Bump httplib2 from 0.18.1 to 0.19.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.18.1 to 0.19.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.18.1...v0.19.0)

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