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

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/passerelle/{env:BRANCH_NAME:}
-envlist = py3-django111-coverage,py3-django22
+envlist = py3-django111-black-coverage,py3-django22
 
 [testenv]
 usedevelop = True
@@ -37,10 +37,12 @@ deps =
   django-ratelimit
   pyquery
   zeep<3.3
+  black: pre-commit
 commands =
   ./get_wcs.sh
   py.test {posargs: {env:FAST:} {env:COVERAGE:} {env:JUNIT:} tests/}
   coverage: ./pylint.sh passerelle/
+  black: pre-commit run black --all-files --show-diff-on-failure
 
 [pytest]
 filterwarnings = default