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/calendar.pyi

@@ -65,6 +65,14 @@ class HTMLCalendar(Calendar):
     def formatmonth(self, theyear: int, themonth: int, withyear: bool = ...) -> str: ...
     def formatyear(self, theyear: int, width: int = ...) -> str: ...
     def formatyearpage(self, theyear: int, width: int = ..., css: Optional[str] = ..., encoding: Optional[str] = ...) -> str: ...
+    if sys.version_info >= (3, 7):
+        cssclasses: List[str]
+        cssclass_today: str
+        cssclasses_weekday_head: List[str]
+        cssclass_month_head: str
+        cssclass_month: str
+        cssclass_year: str
+        cssclass_year_head: str
 
 if sys.version_info < (3, 0):
     class TimeEncoding: