Bump werkzeug from 0.12.2 to 0.15.3

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.12.2 to 0.15.3.
- [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/0.12.2...0.15.3)

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