mute FSTRING_MIDDLE tokens

[asottile]

this is new in 3.12 -- the tests don't yet pass due to pyflakes and pycodestyle. but at least this new test passes:

$ ./venv312/bin/pytest tests/integration/test_plugins.py --show-capture=no
============================= test session starts ==============================
platform linux -- Python 3.12.0b2+, pytest-7.3.2, pluggy-1.0.0
rootdir: /home/asottile/workspace/flake8
configfile: pytest.ini
collected 6 items                                                              

tests/integration/test_plugins.py ......                                 [100%]

============================== 6 passed in 0.17s ===============================