chore(deps): update dependency httplib2 to v0.20.0 (GoogleCloudPlatform#6808)

* chore(deps): update dependency httplib2 to v0.20.0

* Update requirements.txt

Co-authored-by: Bu Sun Kim <[email protected]>

Author: renovate-bot

appengine/standard/mailgun/requirements.txt

@@ -1 +1 @@
-httplib2==0.19.1
+httplib2==0.20.1