Bump httplib2 from 0.9.2 to 0.19.0 in /tools

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.9.2 to 0.19.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@0.9.2...v0.19.0)

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

tools/oldest_constraints.txt

@@ -62,7 +62,7 @@ distro==1.0.1
 # Lexicon oldest constraint is overridden appropriately on relevant DNS provider plugins
 # using their local-oldest-requirements.txt
 dns-lexicon==2.2.1
-httplib2==0.9.2
+httplib2==0.19.0
 idna==2.6
 setuptools==39.0.1
 six==1.11.0