refactored the python cmake module files:

- moved some parts relating to the venv out of PythonCommonMacros to Virtualenv
- removed PythonMacros file
- use an external active env instead of generating a new internal one.
  Also added some checks to avoid a user mixing up different builds within the same env
- finally made some changes on how and when the dune-py module is regenerated

some minor fixes

[testing] ci with index-url set

remove extra-index-url from pip install

add a stopgap solution unti ci repo up and running again

we need pip>=21 so that 'make', 'make install_python', 'make' does not
mess up the packages in an external env (with older versions of pip
the last 'make' wouldn't correctly remove the non editable install).