chore: release 2.2.0 (#872)

:robot: I have created a release \*beep\* \*boop\*
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## [2.2.0](https://www.github.com/googleapis/google-auth-library-python/compare/v2.1.0...v2.2.0) (2021-09-21)


### Features

* add support for workforce pool credentials ([#868](https://www.github.com/googleapis/google-auth-library-python/issues/868)) ([993bab2](https://www.github.com/googleapis/google-auth-library-python/commit/993bab2aaacf3034e09d9f0f25d36c0e815d3a29))
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