Merge Docutils master tests into the main workflow

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,py{38,39,310,311,312},du{18,19}
+envlist = docs,flake8,mypy,twine,py{38,39,310,311,312},du{18,19,-latest}
 isolated_build = True
 
 [testenv]
@@ -19,9 +19,11 @@ passenv =
 description =
     py{38,39,310,311,312}: Run unit tests against {envname}.
     du{18,19}: Run unit tests with the given version of docutils.
+    du-latest: Run unit tests with Docutils' HEAD
 deps =
     du18: docutils==0.18.*
     du19: docutils==0.19.*
+    du-latest: git+https://repo.or.cz/docutils.git#subdirectory=docutils
 extras =
     test
 setenv =
@@ -30,11 +32,6 @@ setenv =
 commands=
     python -X dev -X warn_default_encoding -m pytest --durations 25 {posargs}
 
-[testenv:du-latest]
-commands =
-    python -m pip install "git+https://repo.or.cz/docutils.git#subdirectory=docutils" --no-warn-conflicts
-    {[testenv]commands}
-
 [testenv:flake8]
 basepython = python3
 description =