Introducing option to have locked stable dependencies versions

The idea is to be able to easily lock all, even indirect dependencies by simply installing stqe with pip install stqe[stable].
This can be useful to be able to do repeatable installs like people would for example need when testing z-stream.
You could simply do pip install stqe[stable]==0.2.2 with specific version and have always the same versions of everything.

Going forward, we can build on this and have specific files per OS, e.g. requirements-rhel8, requirements-fedora38...