Test with Docutils 0.19 prereleases (#10598)

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,py{36,37,38,39,310},du{14,15,16,17,18}
+envlist = docs,flake8,mypy,twine,py{36,37,38,39,310},du{14,15,16,17,18,19}
 
 [testenv]
 usedevelop = True
@@ -16,13 +16,14 @@ passenv =
     TERM
 description =
     py{36,37,38,39,310}: Run unit tests against {envname}.
-    du{14,15,16,17,18}: Run unit tests with the given version of docutils.
+    du{14,15,16,17,18,19}: Run unit tests with the given version of docutils.
 deps =
     du14: docutils==0.14.*
     du15: docutils==0.15.*
     du16: docutils==0.16.*
     du17: docutils==0.17.*
     du18: docutils==0.18.*
+    du19: docutils==0.19.*
 extras =
     test
 setenv =
@@ -36,6 +37,11 @@ commands =
     python -m pip install "git+https://repo.or.cz/docutils.git#subdirectory=docutils"
     {[testenv]commands}
 
+[testenv:du19pre]
+commands =
+    python -m pip install --pre "docutils==0.19.*"
+    {[testenv]commands}
+
 [testenv:flake8]
 basepython = python3
 description =