setup: use pytest-black-ng instead of pytest-black

* pytest-black seems out of support. the last commit was from
  2020-10-05. Further pytest-black uses a out of date method which
  mentions pytest>=7.0 with a DeprecationWarning. The simplest solution
  is to use pytest-black-ng which fixes this situation.

setup.cfg

@@ -35,7 +35,7 @@ install_requires =
 
 [options.extras_require]
 tests =
-    pytest-black>=0.3.0
+    pytest-black-ng>=0.4.0
     pytest-invenio>=1.4.2
     Sphinx==4.2.0
 # Kept for backwards compatibility