tox: add black (via pre-commit) to tests (#51394)

.pre-commit-config.yaml

@@ -0,0 +1,8 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+-   repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+    - id: black
+      args: ['--target-version', 'py37', '--skip-string-normalization', '--line-length', '110']

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/docbow/{env:BRANCH_NAME:}
-envlist = py3-{django111,django22}-{pw,pfwb}-coverage,py3-{django111,django22}-mellon-coverage,py3-django11120, py3-{django111,django22}-pfwbmellon-coverage
+envlist = py3-{django111,django22}-{pw,pfwb}-coverage,py3-{django111,django22}-mellon-coverage,py3-django11120, py3-{django111,django22}-pfwbmellon-coverage, black
 
 [testenv]
 usedevelop = True
@@ -23,6 +23,7 @@ deps =
   mellon: django-mellon
   pfwbmellon: django-mellon
   py3: beautifulsoup4
+  pre-commit
 setenv =
   SECRET_KEY=changeme
   pw: PARLEMENT=pw
@@ -41,3 +42,4 @@ commands =
   mellon: py.test {posargs: {env:COVERAGE:} --junitxml=junit-{envname}.xml tests/sso/}
   django11120: py.test {posargs: --junitxml=junit-{envname}.xml tests/main/test_main.py}
   pfwbmellon: py.test {posargs: {env:COVERAGE:} --junitxml=junit-{envname}.xml tests/pfwbmellon/}
+  black: pre-commit run black --all-files --show-diff-on-failure