Make pickle accept IO[str] instead of BinaryIO. (#980)

* Make pickle accept IO[str] instead of BinaryIO.

This makes the following code work:
pickle.Unpickler(cStringIO.StringIO())
(Which didn't work before because cStringIO.StringIO inherits "only"
 from IO[str], not BinaryIO)

* Use bytes instead of str.

stdlib/2/pickle.pyi

@@ -1,14 +1,14 @@
 # Stubs for pickle (Python 2)
 
-from typing import Any, BinaryIO
+from typing import Any, IO
 
 
 HIGHEST_PROTOCOL = ...  # type: int
 
 
-def dump(obj: Any, file: BinaryIO, protocol: int = None) -> None: ...
+def dump(obj: Any, file: IO[bytes], protocol: int = None) -> None: ...
 def dumps(obj: Any, protocol: int = ...) -> bytes: ...
-def load(file: BinaryIO) -> Any: ...
+def load(file: IO[bytes]) -> Any: ...
 def loads(string: bytes) -> Any: ...
 
 
@@ -25,7 +25,7 @@ class UnpicklingError(PickleError):
 
 
 class Pickler:
-    def __init__(self, file: BinaryIO, protocol: int = None) -> None: ...
+    def __init__(self, file: IO[bytes], protocol: int = None) -> None: ...
 
     def dump(self, obj: Any) -> None: ...
 
@@ -33,6 +33,6 @@ class Pickler:
 
 
 class Unpickler:
-    def __init__(self, file: BinaryIO) -> None: ...
+    def __init__(self, file: IO[bytes]) -> None: ...
 
     def load(self) -> Any: ...