Simplified export / import pattern added

PyPNM access scheme was significantly simplified (although old import still work, so there's no need to rewrite old programs).

Docs chaged accordingly.

Build system chaged.

PyPNM for Python 3.4 updated as well.

Wheels attached:

Wheel version	Download link
PyPNM 2.26.23.23 for Python >= 3.10	pypnm-2.26.23.23-py3-none-any.whl
PyPNM 2.26.22.34 for Python >= 3.4	pypnm-2.26.22.34-py3-none-any.whl