Clean up after dropping Python 3.6

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,py{36,37,38,39,310,311},du{14,15,16,17,18,19}
+envlist = docs,flake8,mypy,twine,py{37,38,39,310,311},du{14,15,16,17,18,19}
 isolated_build = True
 
 [testenv]
@@ -16,7 +16,7 @@ passenv =
     EPUBCHECK_PATH
     TERM
 description =
-    py{36,37,38,39,310,311}: Run unit tests against {envname}.
+    py{37,38,39,310,311}: Run unit tests against {envname}.
     du{14,15,16,17,18,19}: Run unit tests with the given version of docutils.
 deps =
     du14: docutils==0.14.*