[th/gitlab-ci-no-el7] drop Python2 support, and CentOS7/el7 and add Fedora40 in gitlab-ci

Maintaining CentOS7 is slightly cumbersome, because it's gcc-4.8 has some issues. Also, currently build on CentOS7 is broken (due to not working with Python2). This would be fixable, but instead drop it.

When no longer testing CentOS7, there is also no more test that tests Python2. At this point, there is no build configuration that does so. Hence, drop Python2 support too.