Use poetry for packaging

[carlio]

This converts prospector into using poetry for packaging instead of setuptools

I created to releases to test here:

• https://github.com/PyCQA/prospector/releases/tag/1.5.0-dev1
• https://github.com/PyCQA/prospector/releases/tag/1.5.0-dev2

Note that I had to drop testing with python 3.10-dev because it cannot install pep8 due to a JSON decode error. I also dropped safety because it is not compatible with the output of pip freeze with the way that poetry sets versions.

(pre-cursor towards #376 )

[carlio]

@Pierre-Sassoulas FYI