tox: add black (via pre-commit) to tests (#50927)

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/fargo/{env:BRANCH_NAME:}
-envlist = py3-dj111-{sqlite,pg}-coverage-pylint,py3-dj22-pg
+envlist = py3-dj111-{sqlite,pg}-black-coverage-pylint,py3-dj22-pg
 
 [testenv]
 usedevelop = True
@@ -29,6 +29,8 @@ deps =
         django-webtest
         WebTest
         mock<4
+        pre-commit
 commands =
         py.test {env:COVERAGE:} {posargs:tests/}
         pylint: ./pylint.sh fargo/
+        black: pre-commit run black --all-files --show-diff-on-failure