Python3: upgrade to 3.9

3.9.7 is currently the latest released one.
As usual, I have downloaded it, checked the GPG signature, and
only then computed the SHA256.

--HG--
branch : heptapod
4 jobs for !48 with topic/heptapod/python-3.9 in 58 minutes and 12 seconds (queued for 3 seconds)
detached
Status Name Job ID Coverage
  Test
passed rspec #254920

00:03:12

 
  Package
failed pkg-ubuntu-focal #254921

00:54:59

 
  Docker
skipped docker-testing #254922
 
  Functional
skipped heptapod-tests-default
 
Name Stage Failure
failed
pkg-ubuntu-focal Package
/builds/heptapod/omnibus-heptapod/lib/gitlab/util.rb:24:in `section'
/builds/heptapod/omnibus-heptapod/lib/gitlab/tasks/build.rake:15:in `block (2 levels) in <top (required)>'
/builds/heptapod/omnibus-heptapod/.bundle/ruby/2.7.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
Tasks: TOP => build:project
(See full trace by running task with --trace)
Cleaning up file based variables
ERROR: Job failed: exit code 1