Bump google-cloud-storage from 1.20.0 to 1.21.0

Bumps [google-cloud-storage](https://github.com/GoogleCloudPlatform/google-cloud-python) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/GoogleCloudPlatform/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/master/CHANGELOG.md)
- [Commits](googleapis/google-cloud-python@storage-1.20.0...storage-1.21.0)

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requirements.txt

@@ -12,7 +12,7 @@ google-auth==1.6.3
 google-auth-httplib2==0.0.3
 google-cloud-core==1.0.3
 google-cloud-firestore==1.5.0
-google-cloud-storage==1.20.0
+google-cloud-storage==1.21.0
 google-resumable-media==0.4.1
 googleapis-common-protos==1.6.0
 grpcio==1.24.3