Switch back to `python-constraints`

[redeboer]

Since #261, QRules runs with python-constraints2 if possible (i.e. for higher versions of Python). This package requires clang however, and is slow in installing, even with uv. It's better to switch to python-constraint2 only once it is not in beta version anymore.

Note that the switch between python-constraint and python-constraints2 does not require any changes to the implementation, see #268.

[redeboer]

Closed by #289