chore: release 1.21.0 (#593)

* chore: updated CHANGELOG.md [ci skip]

* chore: updated setup.cfg [ci skip]

* chore: updated setup.py

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -4,6 +4,13 @@
 
 [1]: https://pypi.org/project/google-auth/#history
 
+## [1.21.0](https://www.github.com/googleapis/google-auth-library-python/compare/v1.20.1...v1.21.0) (2020-08-27)
+
+
+### Features
+
+* add GOOGLE_API_USE_CLIENT_CERTIFICATE support ([#592](https://www.github.com/googleapis/google-auth-library-python/issues/592)) ([c0c995f](https://www.github.com/googleapis/google-auth-library-python/commit/c0c995f3de237a2346b59797ee7c4d44ff2a197c))
+
 ### [1.20.1](https://www.github.com/googleapis/google-auth-library-python/compare/v1.20.0...v1.20.1) (2020-08-06)
 
 

setup.py

@@ -33,7 +33,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.20.1"
+version = "1.21.0"
 
 setup(
     name="google-auth",