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.

testing/python/integration.py

@@ -4,7 +4,7 @@ from _pytest import runner
 
 
 class TestOEJSKITSpecials:
-    def test_funcarg_non_pycollectobj(self, testdir):  # rough jstests usage
+    def test_funcarg_non_pycollectobj(self, testdir, recwarn):  # rough jstests usage
         testdir.makeconftest(
             """
             import pytest
@@ -34,7 +34,7 @@ class TestOEJSKITSpecials:
         pytest._fillfuncargs(clscol)
         assert clscol.funcargs["arg1"] == 42
 
-    def test_autouse_fixture(self, testdir):  # rough jstests usage
+    def test_autouse_fixture(self, testdir, recwarn):  # rough jstests usage
         testdir.makeconftest(
             """
             import pytest