implement some minor 3.7 features (#1966)

References:
- https://docs.python.org/dev/library/datetime.html#datetime.datetime.fromisoformat
- https://docs.python.org/dev/library/crypt.html#crypt.mksalt
- https://docs.python.org/dev/library/contextlib.html#contextlib.AbstractAsyncContextManager
- https://docs.python.org/dev/library/calendar.html#calendar.HTMLCalendar.cssclasses
- https://docs.python.org/dev/library/binascii.html#binascii.b2a_uu

Part of #1965.

stdlib/2and3/contextlib.pyi

@@ -14,6 +14,8 @@ if sys.version_info >= (3, 5):
 
 if sys.version_info >= (3, 6):
     from typing import ContextManager as AbstractContextManager
+if sys.version_info >= (3, 7):
+    from typing import AsyncContextManager as AbstractAsyncContextManager
 
 _T = TypeVar('_T')