gh-69714: Remove unreachable code (and increase test coverage)

This condition cannot be true. `_locale.setlocale()` from the C module
raises `locale.Error` instead of returning `None` for
`different_locale.__enter__` (where `self.oldlocale` is set).

Author: bxsx

Lib/calendar.py

@@ -555,8 +555,6 @@ def __enter__(self):
         _locale.setlocale(_locale.LC_TIME, self.locale)
 
     def __exit__(self, *args):
-        if self.oldlocale is None:
-            return
         _locale.setlocale(_locale.LC_TIME, self.oldlocale)