Exception raised if installed under root.

[Spacetown]

Describe the bug
If black is installed under root and run as restricted user following exception is raised:

python3 -m black --diff doc gcovr
Running chmod with sudoTraceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.6/dist-packages/black/__main__.py", line 3, in <module>
    patched_main()
  File "/usr/local/lib/python3.6/dist-packages/black/__init__.py", line 7002, in patched_main
    main()
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 781, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 700, in make_context
    self.parse_args(ctx, args)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1048, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1630, in handle_parse_result
    value = invoke_param_callback(self.callback, ctx, self, value)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 123, in invoke_param_callback
    return callback(ctx, param, value)
  File "/usr/local/lib/python3.6/dist-packages/black/__init__.py", line 337, in read_pyproject_toml
    value = find_pyproject_toml(ctx.params.get("src", ()))
  File "/usr/local/lib/python3.6/dist-packages/black/__init__.py", line 315, in find_pyproject_toml
    return str(path_user_pyproject_toml) if path_user_pyproject_toml.is_file() else None
  File "/usr/lib/python3.6/pathlib.py", line 1362, in is_file
    return S_ISREG(self.stat().st_mode)
  File "/usr/lib/python3.6/pathlib.py", line 1158, in stat
    return self._accessor.stat(self)
  File "/usr/lib/python3.6/pathlib.py", line 387, in wrapped
    return strfunc(str(pathobj), *args)
PermissionError: [Errno 13] Permission denied: '/root/.config/black'

To Reproduce

1. Install black as root
2. Run python3 -m black --diff .
3. See error (e.g. https://github.com/Spacetown/gcovr/runs/2450843804?check_suite_focus=true)

Expected behavior

The code shall formatted the same way as in `20.8b1, see https://github.com/Spacetown/gcovr/runs/2433190163?check_suite_focus=true.

Environment (please complete the following information):

• Version: 21.4b1
• OS and Python version: Docker image based on ubuntu:18.04 with Python 3.6.7.

Does this bug also happen on master?

This is the current master.

Additional context

[JelleZijlstra]

I think #2158 should fix this, could you check?

[Spacetown]

Sorry, posted in the PR.