Install all dependencies so isort will work correctly.

tox.ini

@@ -23,6 +23,7 @@ deps=
 
     linkcheck,apicheck,configcheck: -r{toxinidir}/requirements/docs.txt
     flake8,flakeplus,pydocstyle: -r{toxinidir}/requirements/pkgutils.txt
+    isort-check: -r{toxinidir}/requirements/test-ci-default.txt
     isort-check: isort
 sitepackages = False
 recreate = False