Bump httplib2 from 0.12.1 to 0.18.0 in /src/emailservice (GoogleCloud…

…Platform#349)

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.12.1 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.12.1...v0.18.0)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/emailservice/requirements.txt

@@ -17,7 +17,7 @@ google-cloud-trace==0.20.2  # via opencensus
 googleapis-common-protos==1.5.5  # via google-api-core
 grpcio-health-checking==1.12.1  # via -r requirements.in
 grpcio==1.16.1            # via -r requirements.in, google-api-core, grpcio-health-checking
-httplib2==0.12.1          # via google-api-python-client, google-auth-httplib2
+httplib2==0.18.0          # via google-api-python-client, google-auth-httplib2
 idna==2.8                 # via requests
 jinja2==2.10              # via -r requirements.in
 markupsafe==1.1.0         # via jinja2