Only install and invoke pytest-black on Python 3

Author: jaraco

pytest.ini

@@ -1,6 +1,6 @@
 [pytest]
 norecursedirs=dist build .tox .eggs
-addopts=--doctest-modules --flake8 --black
+addopts=--doctest-modules --flake8
 doctest_optionflags=ALLOW_UNICODE ELLIPSIS
 filterwarnings=
 	ignore:Possible nested set::pycodestyle:113

setup.cfg

@@ -29,7 +29,7 @@ testing =
 	pytest >= 3.5, !=3.7.3
 	pytest-checkdocs
 	pytest-flake8
-	pytest-black
+	pytest-black; python_version >= "3"
 
 	# local
 

tox.ini

@@ -8,7 +8,8 @@ tox_pip_extensions_ext_venv_update = true
 deps =
 	setuptools>=31.0.1
 commands =
-	pytest {posargs}
+	!py27: pytest --black {posargs}
+	py27: pytest {posargs}
 usedevelop = True
 extras = testing