Resolved tzinfo conflict in tests

[rsb-23]

Fixes #73

Default tzinfo = tzutc(), before running test_pytz_timezone_serializing.
So same value is set in unregister function.

[da4089]

I'm away from my computer for a week. I'll check and merge when I'm home.

Thank you for this!

[da4089]

Did you consider adding a proper deregister_tzid() function which deletes the map entry from __tzidMap? I wonder if that's the better solution here? Or perhaps a general flush_tzid_cache()?

This seems like trying to work around a problem that maybe we can solve directly.

[rsb-23]

This solution only fixes the existing logic for unittest.
This seems sufficient, as there is no issue with actual code and may not occur in real-life scenario.