This would be semver major.

We should verify that clients which cannot move already pin google.auth < 2.0.0dev directly, or transitively.

Extra cleanups:

.- Drop mock wrapper (use from unittest import mock directly). We cannot drop mock because Python 3.6 / 3.7 do not have unittest.mock.AsyncMock in the stdlib.

• Drop six.
• Drop u" prefixes for text strings.