Add PytestRemovedIn9Warning

It's good to have it available already in all 8.* versions, even if it's
not used yet.

doc/en/reference/reference.rst

@@ -1153,6 +1153,9 @@ Custom warnings generated in some situations such as improper usage or deprecate
 .. autoclass:: pytest.PytestRemovedIn8Warning
   :show-inheritance:
 
+.. autoclass:: pytest.PytestRemovedIn9Warning
+  :show-inheritance:
+
 .. autoclass:: pytest.PytestUnhandledCoroutineWarning
    :show-inheritance:
 

src/_pytest/warning_types.py

@@ -56,6 +56,12 @@ class PytestRemovedIn8Warning(PytestDeprecationWarning):
     __module__ = "pytest"
 
 
+class PytestRemovedIn9Warning(PytestDeprecationWarning):
+    """Warning class for features that will be removed in pytest 9."""
+
+    __module__ = "pytest"
+
+
 class PytestReturnNotNoneWarning(PytestRemovedIn8Warning):
     """Warning emitted when a test function is returning value other than None."""
 

src/pytest/__init__.py

@@ -71,6 +71,7 @@ from _pytest.warning_types import PytestConfigWarning
 from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import PytestExperimentalApiWarning
 from _pytest.warning_types import PytestRemovedIn8Warning
+from _pytest.warning_types import PytestRemovedIn9Warning
 from _pytest.warning_types import PytestReturnNotNoneWarning
 from _pytest.warning_types import PytestUnhandledCoroutineWarning
 from _pytest.warning_types import PytestUnhandledThreadExceptionWarning
@@ -131,6 +132,7 @@ __all__ = [
     "PytestDeprecationWarning",
     "PytestExperimentalApiWarning",
     "PytestRemovedIn8Warning",
+    "PytestRemovedIn9Warning",
     "PytestReturnNotNoneWarning",
     "Pytester",
     "PytestPluginManager",