Drop use of 'pytz' in examles / tests

[tseaver]

Except under Python 2.7, current versions of google-api-core and google-cloud-core no longer use or depend on the pytz library, preferring instead to use the stdlib's datetime.timezone and datetime.timezone.utc.

• Add testing with the 2.0.0b1 versions of google-api-core, google-cloud-core, and google-auth under Python 3.9.
• Drop any use of pytz in tests or examples.