Fix type for base64.b64encode, it should accept str or bytes (#736)

Fixes #735

stdlib/3/base64.pyi

@@ -2,10 +2,10 @@
 
 # Based on http://docs.python.org/3.2/library/base64.html
 
-from typing import IO
+from typing import IO, Union
 
 def b64encode(s: bytes, altchars: bytes = ...) -> bytes: ...
-def b64decode(s: bytes, altchars: bytes = ...,
+def b64decode(s: Union[bytes, str], altchars: bytes = ...,
               validate: bool = ...) -> bytes: ...
 def standard_b64encode(s: bytes) -> bytes: ...
 def standard_b64decode(s: bytes) -> bytes: ...