Simplify base64 input and output parameters. (#2587)

This allows passing bytearray() objects to the base64 encode and decode functions, on both Python 2.7 and 3.4.

This also simplifies the code by ignoring 3.2 and 3.3, which are out of scope.

stdlib/2and3/base64.pyi

@@ -4,17 +4,11 @@ from typing import IO, Union, Text
 import sys
 
 if sys.version_info < (3,):
-    _encodable = Union[bytes, Text]
-    _decodable = Union[bytes, Text]
-elif sys.version_info < (3, 3):
-    _encodable = bytes
-    _decodable = bytes
-elif sys.version_info[:2] == (3, 3):
+    _encodable = Union[bytes, unicode]
+    _decodable = Union[bytes, unicode]
+else:
     _encodable = bytes
     _decodable = Union[bytes, str]
-elif sys.version_info >= (3, 4):
-    _encodable = Union[bytes, bytearray, memoryview]
-    _decodable = Union[bytes, bytearray, memoryview, str]
 
 def b64encode(s: _encodable, altchars: bytes = ...) -> bytes: ...
 def b64decode(s: _decodable, altchars: bytes = ...,