Bug#958476: python3-gevent: gevent 1.4 not compatible with python 3.8

[Matthias Klose]

Control: tags -1 + patch

I packaged the recent version, cause the current isn't compatible with 3.9
either. All triggered autopkg tests passed.

Packaging proposal at
https://launchpad.net/ubuntu/+source/python-gevent/20.9.0-0ubuntu1