Stop using poetry as a build backend

[supakeen]

Due to some concerns listed in https://github.com/python-poetry/poetry/issues/3332 poetry does not as of yet support the PEP-621 form of shared metadata information.

As I want to be able to more easily switch build backends in the future let's switch to the pip build backend.

[CAM-Gerlach]

Just FYI—pip is a build frontend, not a backend. Setuptools, flit_core, Hatchling, pdm and poetry are all build backends (though some also offer build frontends too, at least for their own build system).

[supakeen]

Ah, thanks for the clarification :)

[supakeen]

Done and switched to pdm.