[`flake8-bugbear`] Accept immutable slice default arguments (`B008`) (#21823)
Closes issue #21565
## Summary
As pointed out in the issue, slices are currently flagged by B008 but
this behavior is incorrect because slices are immutable.
## Test Plan
Added a test case in the "B006_B008.py" fixture. Sorry for the diff in
the snapshots, the only thing that changes in those flies is the line
numbers, though.
You can also test this manually with this file:
```py
# test_slice.py
def c(d=slice(0, 3)): ...
```
```sh
> target/debug/ruff check tmp/test_slice.py --no-cache --select B008
All checks passed!
```