Tests: linkcheck: ensure Python clock timezone is reset during test teardown

[jayaddison]

Purpose

Resolves a small quirk in the test_too_many_requests_retry_after_HTTP_date test case; after monkeypatching the TZ environment variable and calling time.tzset, the internal state is prepared correctly for each parameterized run of the test case.

The context manager for the monkeypatch clears the TZ environment variable, but doesn't intrinsically know that another time.tzset call is required to properly reset Python's system clock / timezone state.

References

• Relates to linkcheck: Use correct function to convert from UTC time to UNIX epoch #11649.

cc @mitya57

[jayaddison]

Thanks again!