Drop Python 2.7 support #210

This would be semver major (google-api-core 2.0.0). Clients which cannot move forward (e.g, google-cloud-storage already pin google.api_core < 2.0.0dev either directly or transitively (via google-cloud-core).

Extra cleanups:

• Drop mock wrapper (use from unittest import mock directly, replacing work in Use unittest.mock if it is available #208). Can't remove it until we drop support for Python 3.6 / 3.7, as it provides the mock.AsyncMock helper we need.
• Drop six.
• Drop other Python 2.7-specific workarounds (collections_abc, etc.)
• Drop u" prefixes for text strings.
• Drop pytz (per Drop use of pytz #73).
• Replace workaround in bidi.BackgroundConsumer._thread_main with Condition.wait_for. Moved to Replace Python 2.7 workaround in 'bidi.BackgroundConsumer._thread_main' with 'Condition.wait_for' google-cloud-python#15088.