Use declarative metadata

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

tox.ini

@@ -1,6 +1,7 @@
 [tox]
 minversion = 2.4.0
 envlist = docs,flake8,mypy,twine,py{36,37,38,39,310},du{14,15,16,17,18,19}
+isolated_build = True
 
 [testenv]
 usedevelop = True
@@ -34,7 +35,7 @@ commands=
 
 [testenv:du-latest]
 commands =
-    python -m pip install "git+https://repo.or.cz/docutils.git#subdirectory=docutils"
+    python -m pip install "git+https://repo.or.cz/docutils.git#subdirectory=docutils" --no-warn-conflicts
     {[testenv]commands}
 
 [testenv:flake8]
@@ -92,7 +93,7 @@ description =
     Build documentation.
 extras =
     docs
-deps = 
+deps =
     sphinx-autobuild
 commands =
     sphinx-autobuild ./doc ./build/sphinx/
@@ -103,8 +104,9 @@ description =
     Lint package.
 deps =
     twine
+    build
 commands =
-    python setup.py release bdist_wheel sdist
+    python -m build .
     twine check dist/*
 
 [testenv:bindep]