Support pypy for python plugins

[taminob]

The current implementation of PythonPlugin relies on the features and behavior of the Python reference implementation CPython.

To be more flexible, it could be valuable to also support other interpreters like pypy.
Pypy offers a CPython compatibility layer, but unfortunately it is rather incomplete and misses some headers and functions offered in CPython, like:

• Py_NewInterpreter()
• #include <pytypedefs.h>
• Py_EndInterpreter()

This issue is about replacing these and make sure that the library implementation works with pypy.

[taminob]

The library can be compiled with pypy instead of cpython by doing the following steps:

• Install pypy for Python3 (sudo pacman -S pypy3)
• In src/CMakeLists.txt:
  • replace PUBLIC Python::Python by PUBLIC pypy3.10-c
  • add PUBLIC /opt/pypy3/include/pypy3.10 to the target_include_directories

These steps are Arch Linux specific, for the actual integration, a cmake/FindPypy.cmake file should be created.