black need to be installed to launch the full test suite and it's not in the requirements

[Pierre Sassoulas]

When first starting to contribute to django, 205 test errors out (FAILED (errors=205, skipped=1827, expected failures=5)
) with the following stacktrace:

ERROR: test_makemessages_no_settings (i18n.test_extraction.NoSettingsExtractionTests.test_makemessages_no_settings)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/pierre/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/home/pierre/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/unittest/case.py", line 651, in run
    self._callTestMethod(testMethod)
    
  File "/home/pierre/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/unittest/case.py", line 606, in _callTestMethod
    if method() is not None:
    ^^^^^^^^^^^^^^^
  File "/home/pierre/git/django/tests/i18n/test_extraction.py", line 1131, in test_makemessages_no_settings
    out, err = self.run_django_admin(["makemessages", "-l", "en", "-v", "0"])
    ^^^^^^^^^^^^^^^
  File "/home/pierre/git/django/tests/admin_scripts/tests.py", line 167, in run_django_admin
    return self.run_test(
    ^^^^^^^^^^^^^^^
  File "/home/pierre/git/django/tests/admin_scripts/tests.py", line 152, in run_test
    test_environ["PATH"] = self.path_without_formatters
    ^^^^^^^^^^^
  File "/home/pierre/git/django/django/utils/functional.py", line 47, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
    ^^^^^^^^^^^^^^^
  File "/home/pierre/git/django/tests/admin_scripts/tests.py", line 123, in path_without_formatters
    if os.path.commonpath([path_component, formatter_path]) == os.sep
    ^^^^^^^
  File "<frozen posixpath>", line 536, in commonpath
TypeError: expected str, bytes or os.PathLike object, not NoneType

When find_formatters return a black_path that is None (shutil.which("black") is None, meaning black is not installed).

I suggest raising a better error message by catching this in find_formatters directly and/or adding black to tests/requirements/py3.py.