[Security] Bump werkzeug from 0.14.1 to 0.15.3

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.14.1 to 0.15.3. **This update includes a security fix.**
- [Release notes](https://github.com/pallets/werkzeug/releases)
- [Changelog](https://github.com/pallets/werkzeug/blob/master/CHANGES.rst)
- [Commits](https://github.com/pallets/werkzeug/compare/0.14.1...0.15.3

)

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

requirements-dev.txt

@@ -7,4 +7,4 @@ flake8==3.7.9
 responses==0.10.6
 stellar==0.4.5
 wagtail-factories==1.1.0
-Werkzeug==0.14.1
+Werkzeug==0.15.3