Bump werkzeug from 0.8.3 to 0.15.3 in /coinbase_oauth2

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.8.3 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@0.8.3...0.15.3)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

coinbase_oauth2/requirements.txt

@@ -1,7 +1,7 @@
 Flask==0.9
 Jinja2==2.6
 Pygments==1.6
-Werkzeug==0.8.3
+Werkzeug==0.15.3
 httplib2==0.8
 oauth2client==1.1
 requests==2.20.0