Drop ubuntu-20.04 Runner images and setup using pip

[flabbamann]

• ubuntu-20.04 is deprecated, see The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-15 actions/runner-images#11101
• calling setup.py install is also deprecated:

        ********************************************************************************
        Please avoid running ``setup.py`` directly.
        Instead, use pypa/build, pypa/installer or other
        standards-based tools.

        See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
        ********************************************************************************

[coveralls]

coverage: 96.875%. remained the same
when pulling 85ca394 on flabbamann:runner-images
into c7d6da9 on hthiery:master.

[mib1185]

LGTM, thx @flabbamann 👍