Increase minimum Docutils to 0.17

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,py{38,39,310,311},du{14,15,16,17,18,19}
+envlist = docs,flake8,mypy,twine,py{38,39,310,311},du{17,18,19}
 isolated_build = True
 
 [testenv]
@@ -17,11 +17,8 @@ passenv =
     TERM
 description =
     py{38,39,310,311}: Run unit tests against {envname}.
-    du{14,15,16,17,18,19}: Run unit tests with the given version of docutils.
+    du{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.*