Migrate to pyproject.toml

[lovetox]

Closes #288

[lovetox]

I ran the tests, it does not work on python 3.6, because setuptools on that python version does not support editable installs for projects with only pyproject.toml

I see the following options

• Drop python 3.6 compatibility
• Disable the editable install test
• Wait with the move to pyproject.toml

[cvzi]

I think, remove the editable install. I see no reason why the editable install would need to be tested, not sure why I added that in the first place.

[lovetox]

I found more problems, the short story is, setuptools 59 was just not far enough in the implementation for pyproject.toml.
I added a commit to drop python 3.6 compatibility.

I looked at the stats and run them for march 2024

python 3.6 | 0.35% │ 10,700

[cvzi]

I am okay with removing support for Python 3.6.

@TahirJalilov What do you think?

[TahirJalilov]

Thank you for your PR!