chore: release 1.16.1 (#522)
:robot: I have created a release \*beep\* \*boop\*
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### [1.16.1](https://github.com/googleapis/google-auth-library-python/compare/v1.16.0...v1.16.1) (2020-06-04)
### Bug Fixes
* fix impersonated cred exception doc ([#521](https://github.com/googleapis/google-auth-library-python/issues/521)) ([9d5a9a9](https://github.com/googleapis/google-auth-library-python/commit/9d5a9a9884fecbd698a602d2a9fd9bec6b987de7))
* replace environment variable GCE_METADATA_ROOT with GCE_METADATA_HOST ([#433](https://github.com/googleapis/google-auth-library-python/issues/433)) ([8ffb4d3](https://github.com/googleapis/google-auth-library-python/commit/8ffb4d3e832607869026444e5a071c5f3e225fd2)), closes [#339](https://github.com/googleapis/google-auth-library-python/issues/339)
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This PR was generated with [Release Please](https://github.com/googleapis/release-please).
