Bump werkzeug from 0.12 to 0.15.3

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.12 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...0.15.3)

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

requirements.txt

@@ -21,4 +21,4 @@ Sphinx==1.7.1
 sphinx-autodoc-typehints==1.2.5
 treelib==1.3.5
 urwid==1.3.1
-Werkzeug==0.12
+Werkzeug==0.15.3