Improves error message for [except*](https://peps.python.org/pep-0654/)
(Rules: B025, B029, B030, B904)
Example python snippet:
```python
try:
a = 1
except* ValueError:
a = 2
except* ValueError:
a = 2
try:
pass
except* ():
pass
try:
pass
except* 1: # error
pass
try:
raise ValueError
except* ValueError:
raise UserWarning
```
Error messages
Before:
```
$ ruff check --select=B foo.py
foo.py:6:9: B025 try-except block with duplicate exception `ValueError`
foo.py:11:1: B029 Using `except ():` with an empty tuple does not catch anything; add exceptions to handle
foo.py:16:9: B030 `except` handlers should only be exception classes or tuples of exception classes
foo.py:22:5: B904 Within an `except` clause, raise exceptions with `raise ... from err` or `raise ... from None` to distinguish them from errors in exception handling
Found 4 errors.
```
After:
```
$ ruff check --select=B foo.py
foo.py:6:9: B025 try-except* block with duplicate exception `ValueError`
foo.py:11:1: B029 Using `except* ():` with an empty tuple does not catch anything; add exceptions to handle
foo.py:16:9: B030 `except*` handlers should only be exception classes or tuples of exception classes
foo.py:22:5: B904 Within an `except*` clause, raise exceptions with `raise ... from err` or `raise ... from None` to distinguish them from errors in exception handling
Found 4 errors.
```
Closes https://github.com/astral-sh/ruff/issues/14791
---------
Co-authored-by: Micha Reiser <micha@reiser.io>
Fixes#10426
## Summary
Fix rule B030 giving a false positive with Tuple operations like `+`.
[Playground](https://play.ruff.rs/17b086bc-cc43-40a7-b5bf-76d7d5fce78a)
```python
try:
...
except (ValueError,TypeError) + (EOFError,ArithmeticError):
...
```
## Reviewer notes
This is a little more convoluted than I was expecting -- because we can
have valid nested Tuples with operations done on them, the flattening
logic has become a bit more complex.
Shall I guard this behind --preview?
## Test Plan
Unit tested.
