Use a consistent, standard directory for Python packages
We used to ship our own Python packages in /usr/local/lib/python3. This is a non-standard Python directory, which has caused issues before. It also caused issues when I tried to move tailslib there in #16935, which I moved it to /usr/lib/python3 instead.
This change has now been merged into devel, which means that currently we put Python packages into different directories for no apparent reason. I suggest that we move all of them to /usr/lib/python3.
Merge remote-tracking branch 'origin/bugfix/17082-use-consistent-python-packages-dir' into devel (refs: #17082)
Remove obsolete path from PYTHONPATH (refs: #17082)
This partly reverts commit 718a64cb66158d117a7d564ee460f15528507f92.
We moved our Python packages from the non-standard
/usr/local/lib/python3 directory to the standard /usr/lib/python3
directory, so we don't have to set the PYTHONPATH anmore.