Removed dependency on the pytz library.

[spillner]

Most of the Pytz functionality is now available in the standard library in 3.8 and greater, and the Pytz (https://pypi.org/project/pytz/) project has gone into maintenance mode and is no longer recommended for new projects. I noticed that OWSLib was using Pytz only to annotate timestamps as UTC, which doesn't even require the expanded tzinfo support from 3.8/3.9+; the attached patch supports Python 2 (at least Python 2.7) as well.