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/3/unittest/__init__.pyi

@@ -3,12 +3,11 @@
 from typing import (
     Any, Callable, Dict, Iterable, Iterator, List, Optional, Pattern, Sequence,
     Set, FrozenSet, TextIO, Tuple, Type, TypeVar, Union, Generic,
-    overload,
+    overload, ContextManager
 )
 import logging
 import sys
 from types import ModuleType, TracebackType
-from contextlib import ContextManager
 
 
 _T = TypeVar('_T')