bump minimum python version to 3.9

Summary:
And remove some compatibility code.

note that Electrum ABC is currently packaged with 3.9.13

Supported Linux operating systems (core#27483):

FreeBSD 12/13 also ships with 3.9
CentOS-like 8/9 also ships with 3.9 (and 3.11)
OpenSuse Leap also ships with 3.9 (and 3.11)

Debian bullseye or newer
Ubuntu 21.04 and newer

Note that it is relatively easy to install python 3.9 on Ubuntu 20.04 with the package manager:
https://tecadmin.net/how-to-install-python-3-9-on-ubuntu-20-04/

Test Plan:
ninja check-functional

Check the doc for hmac.digest and concurrent.futures to confirm the API is compatible with python 3.9.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15373