recwarn: minor style improvements

In preparation for next commit.

src/_pytest/recwarn.py

@@ -314,7 +314,7 @@ class WarningsChecker(WarningsRecorder):
         ):
             return
 
-        def found_str():
+        def found_str() -> str:
             return pformat([record.message for record in self], indent=2)
 
         try:
@@ -341,14 +341,19 @@ class WarningsChecker(WarningsRecorder):
                         module=w.__module__,
                         source=w.source,
                     )
-            # Check warnings has valid argument type (#10865).
-            wrn: warnings.WarningMessage
-            for wrn in self:
-                self._validate_message(wrn)
 
-    @staticmethod
-    def _validate_message(wrn: Any) -> None:
-        if not isinstance(msg := wrn.message.args[0], str):
-            raise TypeError(
-                f"Warning message must be str, got {msg!r} (type {type(msg).__name__})"
-            )
+            # Currently in Python it is possible to pass other types than an
+            # `str` message when creating `Warning` instances, however this
+            # causes an exception when :func:`warnings.filterwarnings` is used
+            # to filter those warnings. See
+            # https://github.com/python/cpython/issues/103577 for a discussion.
+            # While this can be considered a bug in CPython, we put guards in
+            # pytest as the error message produced without this check in place
+            # is confusing (#10865).
+            for w in self:
+                msg = w.message.args[0]  # type: ignore[union-attr]
+                if isinstance(msg, str):
+                    continue
+                raise TypeError(
+                    f"Warning message must be str, got {msg!r} (type {type(msg).__name__})"
+                )