Bump httplib2 from 0.13.0 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.13.0 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.13.0...v0.18.0)

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

requirements.txt

@@ -15,7 +15,7 @@ entrypoints==0.3
 google-api-python-client==1.7.10
 google-auth==1.6.3
 google-auth-httplib2==0.0.3
-httplib2==0.13.0
+httplib2==0.18.0
 idna==2.8
 idna-ssl==1.1.0
 importlib-metadata==0.18