fixtures: deprecate pytest._fillfuncargs function

This function is exposed and kept alive for the oejskit plugin which is
abandoned and no longer works with recent plugins, so let's prepare to
completely remove it.

src/_pytest/deprecated.py

@@ -25,6 +25,11 @@ FUNCARGNAMES = PytestDeprecationWarning(
     "since pytest 2.3 - use the newer attribute instead."
 )
 
+FILLFUNCARGS = PytestDeprecationWarning(
+    "The `_fillfuncargs` function is deprecated, use "
+    "function._request._fillfixtures() instead if you must."
+)
+
 RESULT_LOG = PytestDeprecationWarning(
     "--result-log is deprecated, please try the new pytest-reportlog plugin.\n"
     "See https://docs.pytest.org/en/latest/deprecations.html#result-log-result-log for more information."

src/_pytest/fixtures.py

@@ -28,6 +28,7 @@ from _pytest.compat import is_generator
 from _pytest.compat import NOTSET
 from _pytest.compat import safe_getattr
 from _pytest.compat import TYPE_CHECKING
+from _pytest.deprecated import FILLFUNCARGS
 from _pytest.deprecated import FIXTURE_POSITIONAL_ARGUMENTS
 from _pytest.deprecated import FUNCARGNAMES
 from _pytest.mark import ParameterSet
@@ -276,6 +277,7 @@ def reorder_items_atscope(items, argkeys_cache, items_by_argkey, scopenum):
 
 def fillfixtures(function):
     """ fill missing funcargs for a test function. """
+    warnings.warn(FILLFUNCARGS, stacklevel=2)
     try:
         request = function._request
     except AttributeError:

src/_pytest/python.py

@@ -1535,7 +1535,7 @@ class Function(PyobjMixin, nodes.Item):
         if isinstance(self.parent, Instance):
             self.parent.newinstance()
             self.obj = self._getobj()
-        fixtures.fillfixtures(self)
+        self._request._fillfixtures()
 
     def _prunetraceback(self, excinfo: ExceptionInfo) -> None:
         if hasattr(self, "_obj") and not self.config.getoption("fulltrace", False):