19.12.2: Bump flask-dance from 2.2.0 to 3.0.0 (#493)

Bumps [flask-dance](https://github.com/singingwolfboy/flask-dance) from 2.2.0 to 3.0.0.
- [Release notes](https://github.com/singingwolfboy/flask-dance/releases)
- [Changelog](https://github.com/singingwolfboy/flask-dance/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/singingwolfboy/flask-dance/compare/v2.2.0...v3.0.0)

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