Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/hmac.pyi

@@ -0,0 +1,44 @@
+import sys
+from _typeshed import ReadableBuffer
+from types import ModuleType
+from typing import Any, AnyStr, Callable, Optional, Union, overload
+
+# TODO more precise type for object of hashlib
+_Hash = Any
+_DigestMod = Union[str, Callable[[], _Hash], ModuleType]
+
+digest_size: None
+
+if sys.version_info >= (3, 8):
+    # In reality digestmod has a default value, but the function always throws an error
+    # if the argument is not given, so we pretend it is a required argument.
+    @overload
+    def new(key: bytes, msg: Optional[ReadableBuffer], digestmod: _DigestMod) -> HMAC: ...
+    @overload
+    def new(key: bytes, *, digestmod: _DigestMod) -> HMAC: ...
+
+elif sys.version_info >= (3, 4):
+    def new(key: bytes, msg: Optional[ReadableBuffer] = ..., digestmod: Optional[_DigestMod] = ...) -> HMAC: ...
+
+else:
+    def new(key: bytes, msg: Optional[ReadableBuffer] = ..., digestmod: Optional[_DigestMod] = ...) -> HMAC: ...
+
+class HMAC:
+    if sys.version_info >= (3,):
+        digest_size: int
+    if sys.version_info >= (3, 4):
+        block_size: int
+        name: str
+    def __init__(self, key: bytes, msg: Optional[ReadableBuffer] = ..., digestmod: _DigestMod = ...) -> None: ...
+    def update(self, msg: ReadableBuffer) -> None: ...
+    def digest(self) -> bytes: ...
+    def hexdigest(self) -> str: ...
+    def copy(self) -> HMAC: ...
+
+@overload
+def compare_digest(__a: ReadableBuffer, __b: ReadableBuffer) -> bool: ...
+@overload
+def compare_digest(__a: AnyStr, __b: AnyStr) -> bool: ...
+
+if sys.version_info >= (3, 7):
+    def digest(key: bytes, msg: ReadableBuffer, digest: str) -> bytes: ...