gh-117655: Prevent `test_strptime` from raising a DeprecationWarning (GH-117668)

* Fix `test_strptime` raises a DeprecationWarning
* Ignore deprecation warnings where appropriate.
* Update Lib/test/datetimetester.py

This is follow on work to silence unnecessary warnings from the test suite that changes for https://github.com/python/cpython/issues/70647 added.