Bump httplib2 from 0.9.2 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.9.2 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@0.9.2...v0.18.0)

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

Author: dependabot[bot]

external_requirements.txt

@@ -10,6 +10,6 @@ enum==0.4.6
 retrying==1.3.3
 pycrypto==2.6.1
 msrestazure==0.6.0
-httplib2==0.9.2
+httplib2==0.18.0
 netaddr==0.7.19
 typing==3.6.4

package/requirements.txt

@@ -14,6 +14,6 @@ enum==0.4.6
 retrying==1.3.3
 pycrypto==2.6.1
 msrestazure==0.6.0
-httplib2==0.9.2
+httplib2==0.18.0
 netaddr==0.7.19
 typing==3.6.4