TST7337: Disallow use of dateutil tzfile objects whose filename contains '.tar.gz'.
TST7337: Fix test failures on windows.

Avoid constructing dateutil timezones explicitly because of filename issue.
Use dateutil.tz.tzutc() to construct UTC timezone instead of
dateutil.tz.gettz('UTC')
Update docs to reflect this.
Tidy up examples in the timezones section of the docs.
Add test for conversion between dateutil.tz.tzutc() and dateutil.tz.gettz('UTC')
Merge pull request #7362 from ahlmss/windows_tests

TST7337: Fix test failures on windows.