Replace deprecated pytest.warns(None)

Replace the deprecated `pytest.warns(None)` use with the `error` warning
filter, as recommended
in pytest-dev/pytest#9404 (comment).
With this change, the tests pass with pytest-8.1.1.

tests/unit/test_call_run.py

@@ -1,3 +1,4 @@
+import warnings
 from unittest.mock import patch
 
 import pytest
@@ -35,11 +36,10 @@ def test_run_with_cfg(self):
     )
     def test_doesnt_warn_about_expected_kwargs(self, expected_kwarg):
         kw = {expected_kwarg: 123}
-        with pytest.warns(None) as warn_rec:
+        with warnings.catch_warnings():
+            warnings.simplefilter("error")
             run("", **kw)
 
-        assert not len(warn_rec)
-
 
 class TestParseGlobalCfg:
     def test_path_correct(self):