.gitlab-ci.yml: Always use Python from PPA as distro package is broken
Fix the following issue by installing the package from deadsnakes PPA:
The following packages have unmet dependencies:
idle-python3.10 : Depends: libjs-mathjax but it is not installable
libpython3.10-testsuite : Depends: net-tools but it is not installable
python3-tk : Depends: blt (>= 2.4z-9) but it is not installable
Depends: libtk8.6 (>= 8.6.0) but it is not installable
Depends: tk8.6-blt2.5 (>= 2.5.3) but it is not installable
E: Unable to correct problems, you have held broken packages.