[packaging] Enforce python >= 3.9 for the debian/ubuntu packages

Summary: The current behavior is to require python3, but this doesn't guarantee that this is an appropriate version. Also Ubuntu Focal ships with 3.8 so we need to install 3.9 explicitely.

Test Plan:

./contrib/release/debian-package.sh -d fabcien

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15496