CI Debian run under pyenv, and other cleanup in gitlab-ci script
CI Debian now runs fully under pyenv. Rationale:
- We ensure consistence by running the entire build process in a virtual environment that has one single Python.
- Specifically, this prevents confusion between the system's Python and pyenv's Python.
- In the future, we can easily generalize this to run CI/CD every night for all target Python version.
Other changes:
- rm 'clang' from name of Debian stance
- rm unused 'native' tag
- rm unused (and misleading) environment variable QTCMAKE
Edited by Wuttke, Joachim