[chore] Release 6.7.0 (firebase#867)

[chore] Release 6.6.0 Take 2 (firebase#830)

[chore] Release 6.5.0 (firebase#779)

[chore] Release 6.4.0 Take 3 (firebase#763)

[chore] Release 6.3.0 (firebase#742)

[chore] Release 6.2.0 (firebase#708)

- Release 6.2.0

[chore] Release 6.1.0 (firebase#672)

- Release 6.1.0

[chore] Release 6.0.1 (firebase#655)

- Release 6.0.1

[chore] Release 6.0.0 (firebase#649)

* [chore] Release 6.0.0

- Major version bump to drop support for Python 3.6
- Added `app_check.verify_token()` API

[chore] Release 5.4.0 (firebase#647)

* [chore] Release 5.4.0

Release v5.4.0
Deprecated Python 3.6 (reached EoL)