chore: release 1.28.0 (#193)

:robot: I have created a release \*beep\* \*boop\*
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## [1.28.0](https://www.github.com/googleapis/python-api-core/compare/v1.27.0...v1.28.0) (2021-05-20)


### Bug Fixes

* require google-auth>=1.25.0 ([#190](https://www.github.com/googleapis/python-api-core/issues/190)) ([155da5e](https://www.github.com/googleapis/python-api-core/commit/155da5e18cc2fdcfa57de6f956b7d078e79cd4b7))


### Miscellaneous Chores

* release 1.28.0 ([#192](https://www.github.com/googleapis/python-api-core/issues/192)) ([11b5da4](https://www.github.com/googleapis/python-api-core/commit/11b5da426a842541ca2b861d3387fc312b3f5b60))
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This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).
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