Datetime test coverage (GH-7544) (GH-7551)

Datetime test coverage (GH-7544) (GH-7551)

* Added a test case for strftime("%z").

The added test checks a case with UTC offest expressed in an integer
number of seconds.

* Added a test comparing naive and aware datetimes.

Check that a greater than  comparison of a naive  datetime instance with
an aware one raises a TypeError.

* Test datetime in fold or in gap comparison both ways.
(cherry picked from commit 4c3e39f61c6a759aa1370497ea3597f3564f9da0)

Co-authored-by: Alexander Belopolsky <abalkin@users.noreply.github.com>