Python: Overhaul path-detection so that only sys.executable is set and PYTHONHOME and even PYTHONPATH aren't set for the python dist.  This results in much simpler logic in the ribbon and installer and is inline with the general advice on stackoverflow to avoid PYTHONHOME.

Based on this, add explicit support for venv: xlOil now looks for the pyvenv.cfg and uses the 'home' specified.

Allow ini-reading logic to handle a single [xlOil_Python.Environment] section as the simplified path logic avoids the stacked env vars we had previously.

Move setting of QT_QPA_PLATFORM_PLUGIN_PATH to qtpy module, with long rant to explain it.

Update docs.

tests: manually checked python 3.6, .3.9, 3.10 with venv and conda env