make PytestReturnNotNoneWarning a RemovedIn8Warning by removing the `@final` decorator

src/_pytest/compat.py

@@ -342,6 +342,12 @@ else:
         return f
 
 
+# TODO: sealed classes are not supported yet
+# https://github.com/mypyc/mypyc/issues/846
+def sealed(f: _T) -> _T:
+    return f
+
+
 if sys.version_info >= (3, 8):
     from functools import cached_property as cached_property
 else:

src/_pytest/warning_types.py

@@ -6,6 +6,7 @@ from typing import TypeVar
 import attr
 
 from _pytest.compat import final
+from _pytest.compat import sealed
 
 
 class PytestWarning(UserWarning):
@@ -48,14 +49,15 @@ class PytestDeprecationWarning(PytestWarning, DeprecationWarning):
     __module__ = "pytest"
 
 
-@final
+@sealed
 class PytestRemovedIn8Warning(PytestDeprecationWarning):
     """Warning class for features that will be removed in pytest 8."""
 
     __module__ = "pytest"
 
 
-class PytestReturnNotNoneWarning(PytestDeprecationWarning):
+@sealed
+class PytestReturnNotNoneWarning(PytestRemovedIn8Warning):
     """Warning emitted when a test function is returning value other than None."""
 
     __module__ = "pytest"