Use explicit directories for style checks

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

tests/test_codestyle.py

@@ -10,11 +10,11 @@
 
 class BaseTests(unittest.TestCase):
     def test_codestyle(self):
-        args = "python -m black --check -l 100 ."
+        args = "python -m black --check -l 100 *.py etc/ src/ tests/ docs"
         try:
             subprocess.check_output(args.split())
         except Exception as e:
             raise Exception(
-                "Black style check failed; please format the code using: "
-                "python -m black --check -l 100"
+                "Black style check failed; please format the code using:\n"
+                "  python -m black -l 100 *.py etc/ src/ tests/ docs"
             ) from e