Enforce black in tox -e check.

Without an argument black just says

    No paths given. Nothing to do 😴

and exits with code 0.

Author: desert

tox.ini

@@ -45,7 +45,7 @@ skip_install = true
 commands =
     python setup.py check --strict --metadata --restructuredtext
     check-manifest {toxinidir}
-    black --check
+    black --check .
     isort --verbose --check-only --diff --recursive src tests setup.py
 
 [testenv:spell]