PytestReturnNotNoneWarning now subclasses PytestRemovedIn8Warning (#10196)

As discussed in #9956 (comment).

Also added PytestRemovedIn8Warning to the reference docs.

Author: graingert

changelog/10196.trivial.rst

@@ -0,0 +1 @@
+:class:`~pytest.PytestReturnNotNoneWarning` is now a subclass of :class:`~pytest.PytestRemovedIn8Warning`: the plan is to make returning non-``None`` from tests an error in the future.

doc/en/reference/reference.rst

@@ -1136,6 +1136,9 @@ Custom warnings generated in some situations such as improper usage or deprecate
 .. autoclass:: pytest.PytestReturnNotNoneWarning
   :show-inheritance:
 
+.. autoclass:: pytest.PytestRemovedIn8Warning
+  :show-inheritance:
+
 .. autoclass:: pytest.PytestUnhandledCoroutineWarning
    :show-inheritance:
 

src/_pytest/warning_types.py

@@ -51,14 +51,13 @@ class PytestDeprecationWarning(PytestWarning, DeprecationWarning):
     __module__ = "pytest"
 
 
-@final
 class PytestRemovedIn8Warning(PytestDeprecationWarning):
     """Warning class for features that will be removed in pytest 8."""
 
     __module__ = "pytest"
 
 
-class PytestReturnNotNoneWarning(PytestDeprecationWarning):
+class PytestReturnNotNoneWarning(PytestRemovedIn8Warning):
     """Warning emitted when a test function is returning value other than None."""
 
     __module__ = "pytest"