Bump httplib2 from 0.18.0 to 0.19.0 in /build

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.18.0 to 0.19.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](https://github.com/httplib2/httplib2/compare/v0.18.0...v0.19.0)

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

build/requirements.txt

@@ -1,2 +1,2 @@
 flake8>=3.5.0
-httplib2==0.18.0
+httplib2==0.19.0