chore: release 0.26.4 (#507)

* updated CHANGELOG.md [ci skip]

* updated setup.py

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

Author: release-please[bot]

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+### [0.26.4](https://www.github.com/googleapis/gapic-generator-python/compare/v0.26.3...v0.26.4) (2020-07-10)
+
+
+### Bug Fixes
+
+* require min google-api-core version of 1.21.0 ([#506](https://www.github.com/googleapis/gapic-generator-python/issues/506)) ([bf787bd](https://www.github.com/googleapis/gapic-generator-python/commit/bf787bd36198288d6a40e45e44e43f0098cfec7c)), closes [#461](https://www.github.com/googleapis/gapic-generator-python/issues/461)
+* tweak oneof detection ([#505](https://www.github.com/googleapis/gapic-generator-python/issues/505)) ([1632e25](https://www.github.com/googleapis/gapic-generator-python/commit/1632e250cfc01a17ccad128c3e065008b334473a))
+
 ### [0.26.3](https://www.github.com/googleapis/gapic-generator-python/compare/v0.26.2...v0.26.3) (2020-07-08)
 
 

setup.py

@@ -20,7 +20,7 @@
 
 PACKAGE_ROOT = os.path.abspath(os.path.dirname(__file__))
 
-version = "0.26.3"
+version = "0.26.4"
 
 with io.open(os.path.join(PACKAGE_ROOT, "README.rst")) as file_obj:
     README = file_obj.read()