Add comments indicating why the exclusions are present

Author: jaraco

setup.cfg

@@ -34,8 +34,10 @@ testing =
 	pytest >= 4.6
 	pytest-checkdocs >= 1.2.3
 	pytest-flake8
+	# python_implementation: workaround for jaraco/skeleton#22
 	pytest-black >= 0.3.7; python_implementation != "PyPy"
 	pytest-cov
+	# python_implementation: workaround for jaraco/skeleton#22
 	pytest-mypy; python_implementation != "PyPy"
 	pytest-enabler