Fixes to ContextManager (#1249)

* add typing.ContextManager for 3.6+ only

This fixes the easier part of #655.

Would it make sense to add a generic typing.ContextManager that exists in any Python version?

* update comment

* fix argument types for ContextManager.__exit__

* add AsyncContextManager

* add @asynccontextmanager

* typing.ContextManager now always exists

* back out async-related changes

Will submit those in a separate PR later

* fix import order

* AbstractContextManager only exists in 3.6+

* AbstractContextManager -> ContextManager

stdlib/2and3/contextlib.pyi

@@ -2,24 +2,23 @@
 
 from typing import (
     Any, Callable, Generator, IO, Iterable, Iterator, Optional, Type,
-    Generic, TypeVar,
+    Generic, TypeVar
 )
 from types import TracebackType
 import sys
+# Aliased here for backwards compatibility; TODO eventually remove this
+from typing import ContextManager as ContextManager
+
+if sys.version_info >= (3, 6):
+    from typing import ContextManager as AbstractContextManager
 
 _T = TypeVar('_T')
+
 _ExitFunc = Callable[[Optional[Type[BaseException]],
-                      Optional[Exception],
+                      Optional[BaseException],
                       Optional[TracebackType]], bool]
 _CM_EF = TypeVar('_CM_EF', ContextManager, _ExitFunc)
 
-# TODO already in PEP, have to get added to mypy
-class ContextManager(Generic[_T]):
-    def __enter__(self) -> _T: ...
-    def __exit__(self, exc_type: Optional[Type[BaseException]],
-                 exc_val: Optional[Exception],
-                 exc_tb: Optional[TracebackType]) -> bool: ...
-
 if sys.version_info >= (3, 2):
     class GeneratorContextManager(Generic[_T], ContextManager[_T]):
         def __call__(self, func: Callable[..., _T]) -> Callable[..., _T]: ...