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/binascii.pyi

@@ -21,7 +21,10 @@ else:
     _Ascii = Union[bytes, Text]
 
 def a2b_uu(string: _Ascii) -> bytes: ...
-def b2a_uu(data: _Bytes) -> bytes: ...
+if sys.version_info >= (3, 7):
+    def b2a_uu(data: _Bytes, *, backtick: bool = ...) -> bytes: ...
+else:
+    def b2a_uu(data: _Bytes) -> bytes: ...
 def a2b_base64(string: _Ascii) -> bytes: ...
 if sys.version_info >= (3, 6):
     def b2a_base64(data: _Bytes, *, newline: bool = ...) -> bytes: ...