Use declarative metadata

- Move to pyproject.toml metadata
- Update references to `setup.py`
- Use pypa/build
- Update workflows and tooling

.github/workflows/docutils-latest.yml

@@ -21,8 +21,6 @@ jobs:
         python-version: 3
     - name: Check Python version
       run: python --version
-    - name: Unpin docutils
-      run: sed -i -e "s/'docutils>=.*'/'docutils'/" setup.py
     - name: Install graphviz
       run: sudo apt-get install graphviz
     - name: Install dependencies

.github/workflows/lint.yml

@@ -20,6 +20,6 @@ jobs:
       with:
         python-version: 3
     - name: Install dependencies
-      run:  pip install -U tox
+      run:  python -m pip install -U tox pip
     - name: Run Tox
       run:  tox -e ${{ matrix.tool }}