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/datetime.pyi

@@ -223,6 +223,9 @@ class datetime:
     else:
         @classmethod
         def combine(cls, date: date, time: time) -> datetime: ...
+    if sys.version_info >= (3, 7):
+        @classmethod
+        def fromisoformat(cls, date_string: str) -> datetime: ...
     def strftime(self, fmt: _Text) -> str: ...
     if sys.version_info >= (3,):
         def __format__(self, fmt: str) -> str: ...