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] <support@github.com>

requirements.txt

@@ -22,7 +22,7 @@ googleapis-common-protos==1.51.0
 grpc-google-iam-v1==0.12.3
 grpcio==1.26.0
 hdfs==2.5.8
-httplib2==0.12.0
+httplib2==0.18.0
 idna==2.8
 mock==2.0.0
 monotonic==1.5