Filter SyntaxWarnings during AST parsing

[cdce8p]

Especially with PEP 765 in 3.14, Python has been getting more liberal in emitting SyntaxWarnings during AST parsing and compilation. Generally, they aren't really helpful for mypy itself. There are open discussions to add a flag which would disable these. Until that's implemented, filter the warnings manually.

This also get's rid of the warnings emitted on test code. If at some point return in finally will be made an error, those tests could be adjust / removed. Until then, we can continue to test these as is without issues.

def func() -> None:
    try:
        x = 1/0
    finally:
        return None  # return in finally

"Hello \P world"  # invalid escape sequence
"" is 1            # "is" with 'int' literal

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
- python/pyspark/pandas/supported_api_gen.py:400: SyntaxWarning: invalid escape sequence '\_'
-   return func_str[:-1] + "\_"  # noqa: W605