Bump google-auth from 1.27.1 to 1.28.0 (#4076)

Bumps [google-auth](https://github.com/googleapis/google-auth-library-python) from 1.27.1 to 1.28.0.
- [Release notes](https://github.com/googleapis/google-auth-library-python/releases)
- [Changelog](https://github.com/googleapis/google-auth-library-python/blob/master/CHANGELOG.md)
- [Commits](googleapis/google-auth-library-python@v1.27.1...v1.28.0)

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requirements.txt

@@ -54,7 +54,7 @@ Python-jose==3.2.0
 gitpython==3.1.14
 ptvsd==4.3.2
 python-gitlab==2.6.0
-google-auth==1.27.1
+google-auth==1.28.0
 google-api-python-client==1.12.8
 google-auth-oauthlib==0.4.3
 drf_yasg2==1.19.4