Move the metadata from setup.py into setup.cfg.

Added `pyproject.toml`.
Removed `setup.py` - it is no longer needed.
Got rid of tests erroroneously finding their way into the wheel.

.github/workflows/python-publish.yml

@@ -21,11 +21,11 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install setuptools wheel twine
+        pip install --upgrade setuptools wheel build twine
     - name: Build and publish
       env:
         TWINE_USERNAME: ${{ secrets.PYPI_USERNAME }}
         TWINE_PASSWORD: ${{ secrets.PYPI_PASSWORD }}
       run: |
-        python setup.py sdist bdist_wheel
+        python -m build -nwsx .
         twine upload dist/*