[tox] configcheck needs to install doc requirements

tox.ini

@@ -10,7 +10,7 @@ deps=
     3.4,3.5: -r{toxinidir}/requirements/test-ci-default.txt
     pypy,pypy3: -r{toxinidir}/requirements/test-ci-base.txt
 
-    linkcheck,apicheck: -r{toxinidir}/requirements/docs.txt
+    linkcheck,apicheck,configcheck: -r{toxinidir}/requirements/docs.txt
     flake8,flakeplus: -r{toxinidir}/requirements/pkgutils.txt
 sitepackages = False
 recreate = False