Update Python base version from 3.6 to 3.8. For Zed the minimum
Python runtime versions are Python 3.8 and 3.9 [1]. As a third
Python version I also added 3.10 already.
Removed ThreadedHTTPServer and replaced it with ThreadingHTTPServer from
standard library, which is new in Python 3.7.
[1]: https://governance.openstack.org/tc/reference/runtimes/zed.html
Change-Id: I035c569b4feaa23b00c0cad50c36aaecf06f7848
737523f3bc