Fix blake2 binding (#1663)

* Fix blake2 binding

Currently calling `hashlib.blake2b` results in the following type errors:

Cannot instantiate abstract class '_BlakeHash' with abstract attributes 'copy', 'digest', 'hexdigest' and 'update'
Missing positional arguments "data", "key", "salt", "person", "fanout", "depth", "leaf_size", "node_offset", "node_depth", "inner_size", "last_node" in call to "_BlakeHash"

* Additional changes to reflect the hashlib implementation

Modifies the type signatures of:
 * blake2b
 * blake2s
 * sha3_224
 * sha3_256
 * sha3_384
 * sha3_512
 * shake_128
 * shake_256
To reflect the types that are implemented in the standard library.
These should be exposed as `type`s instead of `builtin_function_or_method`s.
e.g.
In [40]: type(hashlib.blake2b)
Out[40]: type

In [41]: type(hashlib.md5)
Out[41]: builtin_function_or_method

stdlib/3/hashlib.pyi

@@ -1,12 +1,11 @@
 # Stubs for hashlib
 
 import sys
-from abc import abstractmethod, ABCMeta
 from typing import AbstractSet, Optional, Union
 
 _DataType = Union[bytes, bytearray, memoryview]
 
-class _Hash(metaclass=ABCMeta):
+class _Hash(object):
     digest_size = ...  # type: int
     block_size = ...  # type: int
 
@@ -15,14 +14,12 @@ class _Hash(metaclass=ABCMeta):
     # formally specified, so may not exist on some platforms
     name = ...  # type: str
 
-    @abstractmethod
-    def update(self, arg: _DataType) -> None: ...
-    @abstractmethod
-    def digest(self) -> bytes: ...
-    @abstractmethod
-    def hexdigest(self) -> str: ...
-    @abstractmethod
+    def __init__(self, data: _DataType = ...) -> None: ...
+
     def copy(self) -> _Hash: ...
+    def digest(self) -> bytes: ...
+    def hexdigest(self) -> str: ...
+    def update(self, arg: _DataType) -> None: ...
 
 def md5(arg: _DataType = ...) -> _Hash: ...
 def sha1(arg: _DataType = ...) -> _Hash: ...
@@ -42,27 +39,24 @@ if sys.version_info >= (3, 4):
     def pbkdf2_hmac(hash_name: str, password: _DataType, salt: _DataType, iterations: int, dklen: Optional[int] = ...) -> bytes: ...
 
 if sys.version_info >= (3, 6):
-    class _VarLenHash(metaclass=ABCMeta):
+    class _VarLenHash(object):
         digest_size = ...  # type: int
         block_size = ...  # type: int
         name = ...  # type: str
 
-        @abstractmethod
-        def digest(self, length: int) -> bytes: ...
-        @abstractmethod
-        def hexdigest(self, length: int) -> str: ...
-        @abstractmethod
-        def update(self, arg: _DataType) -> None: ...
-        @abstractmethod
+        def __init__(self, data: _DataType = ...) -> None: ...
+
         def copy(self) -> _VarLenHash: ...
+        def digest(self, length: int) -> bytes: ...
+        def hexdigest(self, length: int) -> str: ...
+        def update(self, arg: _DataType) -> None: ...
 
-    def sha3_224(arg: _DataType = ...) -> _Hash: ...
-    def sha3_256(arg: _DataType = ...) -> _Hash: ...
-    def sha3_384(arg: _DataType = ...) -> _Hash: ...
-    def sha3_512(arg: _DataType = ...) -> _Hash: ...
-
-    def shake_128(arg: _DataType = ...) -> _VarLenHash: ...
-    def shake_256(arg: _DataType = ...) -> _VarLenHash: ...
+    sha3_224 = _Hash
+    sha3_256 = _Hash
+    sha3_384 = _Hash
+    sha3_512 = _Hash
+    shake_128 = _VarLenHash
+    shake_256 = _VarLenHash
 
     def scrypt(password: _DataType, *, salt: _DataType, n: int, r: int, p: int, maxmem: int = ..., dklen: int = ...) -> bytes: ...
 
@@ -72,7 +66,7 @@ if sys.version_info >= (3, 6):
         PERSON_SIZE = ...  # type: int
         SALT_SIZE = ...  # type: int
 
-        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: _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: ...
 
     blake2b = _BlakeHash
     blake2s = _BlakeHash