test/test_checks.py: flake8 test relies on flake8-quotes extension being present

[mika]

This was reported as https://bugs.debian.org/1080108 in Debian, quoting the relevant build failure:

FAIL: test_stdin (test.test_checks.TestFlake8Stdin.test_stdin)
Test using stdin.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/.pybuild/cpython3_3.12/build/test/test_checks.py", line 24, in test_stdin
    self.assertEqual(len(stdout_lines), 3)
AssertionError: 0 != 3

This is caused by executing flake8 -select=Q - < data/doubles.py without having the flake8-quotes extension explicitly available at this stage.

I provided a MR for the Debian packaging at https://salsa.debian.org/python-team/packages/flake8-quotes/-/merge_requests/1 and will provide a PR here which makes sure to run flake8 with a custom configuration which makes sure the flake8-quotes package is found.