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/base64.pyi

@@ -0,0 +1,41 @@
+import sys
+from typing import IO, Optional, Union
+
+if sys.version_info >= (3, 0):
+    _encodable = bytes
+    _decodable = Union[bytes, str]
+else:
+    _encodable = Union[bytes, unicode]
+    _decodable = Union[bytes, unicode]
+
+def b64encode(s: _encodable, altchars: Optional[bytes] = ...) -> bytes: ...
+def b64decode(s: _decodable, altchars: Optional[bytes] = ..., validate: bool = ...) -> bytes: ...
+def standard_b64encode(s: _encodable) -> bytes: ...
+def standard_b64decode(s: _decodable) -> bytes: ...
+def urlsafe_b64encode(s: _encodable) -> bytes: ...
+def urlsafe_b64decode(s: _decodable) -> bytes: ...
+def b32encode(s: _encodable) -> bytes: ...
+def b32decode(s: _decodable, casefold: bool = ..., map01: Optional[bytes] = ...) -> bytes: ...
+def b16encode(s: _encodable) -> bytes: ...
+def b16decode(s: _decodable, casefold: bool = ...) -> bytes: ...
+
+if sys.version_info >= (3, 10):
+    def b32hexencode(s: _encodable) -> bytes: ...
+    def b32hexdecode(s: _decodable, casefold: bool = ...) -> bytes: ...
+
+if sys.version_info >= (3, 4):
+    def a85encode(b: _encodable, *, foldspaces: bool = ..., wrapcol: int = ..., pad: bool = ..., adobe: bool = ...) -> bytes: ...
+    def a85decode(b: _decodable, *, foldspaces: bool = ..., adobe: bool = ..., ignorechars: Union[str, bytes] = ...) -> bytes: ...
+    def b85encode(b: _encodable, pad: bool = ...) -> bytes: ...
+    def b85decode(b: _decodable) -> bytes: ...
+
+def decode(input: IO[bytes], output: IO[bytes]) -> None: ...
+def encode(input: IO[bytes], output: IO[bytes]) -> None: ...
+
+if sys.version_info >= (3,):
+    def encodebytes(s: bytes) -> bytes: ...
+    def decodebytes(s: bytes) -> bytes: ...
+
+if sys.version_info < (3, 9):
+    def encodestring(s: bytes) -> bytes: ...
+    def decodestring(s: bytes) -> bytes: ...