Bump google-auth from 1.27.1 to 1.28.0

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)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -5,7 +5,7 @@ codecov==2.1.11
 configobj==5.0.6
 coverage==5.5
 dpath==2.0.1
-google-auth==1.27.1
+google-auth==1.28.0
 httpretty==1.0.5
 idna==2.10
 kubernetes==12.0.1