Filter SyntaxWarnings during AST parsing (#20023)

Especially with [PEP 765](https://peps.python.org/pep-0765/) 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._

```py
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
```

Author: cdce8p

mypy/fastparse.py

@@ -232,9 +232,12 @@ def parse(
         assert options.python_version[0] >= 3
         feature_version = options.python_version[1]
     try:
-        # Disable deprecation warnings about \u
+        # Disable
+        # - deprecation warnings about \u
+        # - syntax warnings for 'invalid escape sequence' (3.12+)  and 'return in finally' (3.14+)
         with warnings.catch_warnings():
             warnings.filterwarnings("ignore", category=DeprecationWarning)
+            warnings.filterwarnings("ignore", category=SyntaxWarning)
             ast = ast3_parse(source, fnam, "exec", feature_version=feature_version)
 
         tree = ASTConverter(

pyproject.toml

@@ -227,9 +227,6 @@ xfail_strict = true
 # Force warnings as errors
 filterwarnings = [
   "error",
-  # Some testcases may contain code that emits SyntaxWarnings, and they are not yet
-  # handled consistently in 3.14 (PEP 765)
-  "default::SyntaxWarning",
 ]
 
 [tool.coverage.run]