fallback to pure python implementation if used on a non-supported platform

[Girgitt]

On Windows and Raspberry PI PJON-python uses C++ client compiled from PJON sources to maintain compatibility with PJON protocol.

The C++ client is compiled and provided with PJON-python for Win and RPI platforms.

For other platforms (e.g. Linux) PJON-python should use older python-only implementation but should warn the compatibility is provided up to PJON v5 only.