Use the ReadableBuffer type in more places (#4245)

This is a follow-up on #4232. memoryview, hashlib, and hmac are updated
to use ReadableBuffer type instead of their own home-spun unions of
bytes, bytearray and whatever else each use case used. mmap is being
handled in #4244, and I'll leave BinaryIO for another day (or possibly
another person) because it's going to require some messy code
duplication because the relevant methods are defined in IO[AnyStr].

There's one corner case I'm not quite sure how best to handle: the
documentation for hmac.digest claim that the parmaeters have the same
meanings as in hmac.new, but in CPython the latter has an explicit check
that `key` is bytes or bytearray while the former works with a
memory-view. For now I've matched the documentation.

Also, the documentation for HMAC.update says that `msg` can be any type
supported by hashlib from Python 3.4; but I can't see anything in the
Python 2.7 implementation that would prevent it also taking bytes-like
objects, so I've not tried to treat Python 2 any different to Python 3.

stdlib/3/hashlib.pyi

@@ -2,8 +2,7 @@
 
 import sys
 from typing import AbstractSet, Optional, Union
-
-_DataType = Union[bytes, bytearray, memoryview]
+from _typeshed import ReadableBuffer
 
 class _Hash(object):
     digest_size: int
@@ -14,41 +13,41 @@ class _Hash(object):
     # formally specified, so may not exist on some platforms
     name: str
 
-    def __init__(self, data: _DataType = ...) -> None: ...
+    def __init__(self, data: ReadableBuffer = ...) -> None: ...
 
     def copy(self) -> _Hash: ...
     def digest(self) -> bytes: ...
     def hexdigest(self) -> str: ...
-    def update(self, __data: _DataType) -> None: ...
+    def update(self, __data: ReadableBuffer) -> None: ...
 
 if sys.version_info >= (3, 9):
-    def md5(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
-    def sha1(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
-    def sha224(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
-    def sha256(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
-    def sha384(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
-    def sha512(string: _DataType = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def md5(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def sha1(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def sha224(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def sha256(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def sha384(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
+    def sha512(string: ReadableBuffer = ..., *, usedforsecurity: bool = ...) -> _Hash: ...
 elif sys.version_info >= (3, 8):
-    def md5(string: _DataType = ...) -> _Hash: ...
-    def sha1(string: _DataType = ...) -> _Hash: ...
-    def sha224(string: _DataType = ...) -> _Hash: ...
-    def sha256(string: _DataType = ...) -> _Hash: ...
-    def sha384(string: _DataType = ...) -> _Hash: ...
-    def sha512(string: _DataType = ...) -> _Hash: ...
+    def md5(string: ReadableBuffer = ...) -> _Hash: ...
+    def sha1(string: ReadableBuffer = ...) -> _Hash: ...
+    def sha224(string: ReadableBuffer = ...) -> _Hash: ...
+    def sha256(string: ReadableBuffer = ...) -> _Hash: ...
+    def sha384(string: ReadableBuffer = ...) -> _Hash: ...
+    def sha512(string: ReadableBuffer = ...) -> _Hash: ...
 else:
-    def md5(__string: _DataType = ...) -> _Hash: ...
-    def sha1(__string: _DataType = ...) -> _Hash: ...
-    def sha224(__string: _DataType = ...) -> _Hash: ...
-    def sha256(__string: _DataType = ...) -> _Hash: ...
-    def sha384(__string: _DataType = ...) -> _Hash: ...
-    def sha512(__string: _DataType = ...) -> _Hash: ...
+    def md5(__string: ReadableBuffer = ...) -> _Hash: ...
+    def sha1(__string: ReadableBuffer = ...) -> _Hash: ...
+    def sha224(__string: ReadableBuffer = ...) -> _Hash: ...
+    def sha256(__string: ReadableBuffer = ...) -> _Hash: ...
+    def sha384(__string: ReadableBuffer = ...) -> _Hash: ...
+    def sha512(__string: ReadableBuffer = ...) -> _Hash: ...
 
-def new(name: str, data: _DataType = ...) -> _Hash: ...
+def new(name: str, data: ReadableBuffer = ...) -> _Hash: ...
 
 algorithms_guaranteed: AbstractSet[str]
 algorithms_available: AbstractSet[str]
 
-def pbkdf2_hmac(hash_name: str, password: _DataType, salt: _DataType, iterations: int, dklen: Optional[int] = ...) -> bytes: ...
+def pbkdf2_hmac(hash_name: str, password: ReadableBuffer, salt: ReadableBuffer, iterations: int, dklen: Optional[int] = ...) -> bytes: ...
 
 if sys.version_info >= (3, 6):
     class _VarLenHash(object):
@@ -56,12 +55,12 @@ if sys.version_info >= (3, 6):
         block_size: int
         name: str
 
-        def __init__(self, data: _DataType = ...) -> None: ...
+        def __init__(self, data: ReadableBuffer = ...) -> None: ...
 
         def copy(self) -> _VarLenHash: ...
         def digest(self, __length: int) -> bytes: ...
         def hexdigest(self, __length: int) -> str: ...
-        def update(self, __data: _DataType) -> None: ...
+        def update(self, __data: ReadableBuffer) -> None: ...
 
     sha3_224 = _Hash
     sha3_256 = _Hash
@@ -70,7 +69,7 @@ if sys.version_info >= (3, 6):
     shake_128 = _VarLenHash
     shake_256 = _VarLenHash
 
-    def scrypt(password: _DataType, *, salt: Optional[_DataType] = ..., n: Optional[int] = ..., r: Optional[int] = ..., p: Optional[int] = ..., maxmem: int = ..., dklen: int = ...) -> bytes: ...
+    def scrypt(password: ReadableBuffer, *, salt: Optional[ReadableBuffer] = ..., n: Optional[int] = ..., r: Optional[int] = ..., p: Optional[int] = ..., maxmem: int = ..., dklen: int = ...) -> bytes: ...
 
     class _BlakeHash(_Hash):
         MAX_DIGEST_SIZE: int
@@ -79,9 +78,9 @@ if sys.version_info >= (3, 6):
         SALT_SIZE: int
 
         if sys.version_info >= (3, 9):
-            def __init__(self, __data: _DataType = ..., *, digest_size: int = ..., key: _DataType = ..., salt: _DataType = ..., person: _DataType = ..., fanout: int = ..., depth: int = ..., leaf_size: int = ..., node_offset: int = ..., node_depth: int = ..., inner_size: int = ..., last_node: bool = ..., usedforsecurity: bool = ...) -> None: ...
+            def __init__(self, __data: ReadableBuffer = ..., *, digest_size: int = ..., key: ReadableBuffer = ..., salt: ReadableBuffer = ..., person: ReadableBuffer = ..., fanout: int = ..., depth: int = ..., leaf_size: int = ..., node_offset: int = ..., node_depth: int = ..., inner_size: int = ..., last_node: bool = ..., usedforsecurity: bool = ...) -> None: ...
         else:
-            def __init__(self, __data: _DataType = ..., *, digest_size: int = ..., key: _DataType = ..., salt: _DataType = ..., person: _DataType = ..., fanout: int = ..., depth: int = ..., leaf_size: int = ..., node_offset: int = ..., node_depth: int = ..., inner_size: int = ..., last_node: bool = ...) -> None: ...
+            def __init__(self, __data: ReadableBuffer = ..., *, digest_size: int = ..., key: ReadableBuffer = ..., salt: ReadableBuffer = ..., person: ReadableBuffer = ..., fanout: int = ..., depth: int = ..., leaf_size: int = ..., node_offset: int = ..., node_depth: int = ..., inner_size: int = ..., last_node: bool = ...) -> None: ...
 
     blake2b = _BlakeHash
     blake2s = _BlakeHash