Bump httplib2 from 0.14.0 to 0.19.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.14.0 to 0.19.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.14.0...v0.19.0)

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

requirements.txt

@@ -24,7 +24,7 @@ Flask==1.1.2
 Flask-WTF==0.14.3
 future==0.18.2
 gpg===1.13.1-unknown
-httplib2==0.14.0
+httplib2==0.19.0
 idna==2.8
 itsdangerous==1.1.0
 Jinja2==2.11.2