Proposal to fix #373 (Use hatchling build backend) and #374 (Update Towncrier config in pyproject.toml)

I have moved everything except for flake8 settings and pycodestyle settings from setup.cfg to pyproject.toml

I have prepared these scripts:

• style:fmt runs Black and isort (all files now have imports sorted by isort)
• style:lint runs flake8 and pycodestyle
• docs:build runs pdoc3 to build API documentation in the api-docs directory (added to .gitignore
• docs:serve runs pdoc3 local server on localhost:8080 to observe API docs updated live as you update docstrings. However, this somehow does not work in this project and I don't know exactly how to solve the error.
• test:cov runs pytest-cov
• test:no-cov runs pytest with --no-cov
• test:tox_test runs tox

Scripts can be run with hatch run, e.g. hatch.run style:fmt. All the test scripts fail because of python-semver\tests\coerce.py not being a proper Python file (invalid syntax). I don't know what purpose of this file is, so I did not touch it. It makes tests fail even if you run then natively with tox, so repairing this is beyond the scope of this PR, methinks.

I changed the classifier Environment :: Web Environment to Environment :: Console as I think it's more fitting.

I also removed Python 3.6 from black's matxix in anticipation of #372 (Remove support of Python 3.6)