Add inital CI for 3.14 (#4804)

* Add inital CI for 3.14

- State in pyproject that we somewhat work in 3.14
- Enable py314 tox
- Add base testing action so we don't regress

**Note:** We don't support some 3.14 grammar/parsing still

Test:
- Install in 3.14 and format something
```
MacBookPro:black cooper$ python3 -V
Python 3.14.0
MacBookPro:black cooper$ python3 -m venv --upgrade-deps /tmp/tb
t/mp/MacBookPro:black cooper$ /tmp/tb/bin/pip install -e .
...
MacBookPro:black cooper$ time /tmp/tb/bin/black ../bandersnatch/ ../flake8-bugbear/
All done! ✨ 🍰 ✨
134 files left unchanged.

real    0m0.189s
user    0m0.159s
sys    0m0.036s
```
- Run linting/tests:
- `/tmp/tb/bin/tox -e py314`
- `/tmp/tb/bin/pre-commit run -a`
  - TODO: flake8 + mypy still do not pass in 3.14
  - Let's do in dedicated PR later

* Add CHANGES.md entries