Bump httplib2 from 0.9.2 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.9.2 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](https://github.com/httplib2/httplib2/compare/0.9.2...v0.18.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -9,6 +9,6 @@ python-socketio==1.4.4
 PyYAML==4.2b1
 requests==2.20.0
 Werkzeug==0.11.10
-httplib2==0.9.2
+httplib2==0.18.0
 flask-classy==0.6.10
 GitPython==2.1.3