Bump httplib2 from 0.12.3 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.12.3 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/v0.12.3...v0.18.0)

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

requirements.txt

@@ -4,7 +4,7 @@ cffi==1.12.3
 chardet==3.0.4
 gevent==1.4.0
 greenlet==0.4.15
-httplib2==0.12.3
+httplib2==0.18.0
 idna==2.8
 pycparser==2.19
 python-mpd2==1.0.0