Bump httplib2 from 0.12.0 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.12.0 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.12.0...v0.18.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -13,7 +13,7 @@ django-rest-framework-social-oauth2==1.0.4
 djangorestframework==3.9.1
 html2text==2018.1.9
 html2markdown==0.1.7
-httplib2==0.12.0
+httplib2==0.18.0
 idna==2.8
 mistletoe==0.7.1
 oauth2==1.9.0.post1