Bump werkzeug from 0.11.3 to 0.15.3

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.11.3 to 0.15.3.
- [Release notes](https://github.com/pallets/werkzeug/releases)
- [Changelog](https://github.com/pallets/werkzeug/blob/master/CHANGES.rst)
- [Commits](pallets/werkzeug@0.11.3...0.15.3)

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

requirements.txt

@@ -44,6 +44,6 @@ sphinx-rtd-theme==0.1.6
 SQLAlchemy==1.0.9
 translate-toolkit==1.13.0
 WebOb==1.3.1
-Werkzeug==0.11.3
+Werkzeug==0.15.3
 wheel==0.24.0
 WTForms==2.0.1