Compatibility with pybuild/pyproject/pip

Looks like the python build strategy shifted towards using pip, and largely switched from setup.py to pyproject.toml .

Myself, I couldn't yet find a (pretty nor hackish) way to make 'pip install' work with crossenv, without using external pip packages (=offline), for example to build libselinux using their Makefile. The old way using setup.py still works though...

If someone is able to use this tool combination, please comment...