Drop Python 2.7 support.

[tseaver]

This would be semver major, including moving to depend on google-api-core 2.0.0 (see: googleapis/python-api-core#210).

Clients which cannot move forward (e.g, google-cloud-storage) already pin google.cloud.core < 2.0.0dev directly.

Extra cleanups:

• Drop mock wrapper (use from unittest import mock directly).
• Drop six.
• Drop u" prefixes for text strings.
• Drop pytz (as in Drop use of pytz python-api-core#73).