Bump Docutils dependency

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,coverage,py{36,37,38,39,310},du{14,15,16,17}
+envlist = docs,flake8,mypy,twine,coverage,py{36,37,38,39,310},du{14,15,16,17,18}
 
 [testenv]
 usedevelop = True
@@ -16,12 +16,13 @@ passenv =
     TERM
 description =
     py{36,37,38,39,310}: Run unit tests against {envname}.
-    du{12,13,14}: Run unit tests with the given version of docutils.
+    du{14,15,16,17,18}: 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.*
 extras =
     test
 setenv =