test: keep mutation guard Python 3.10 compatible

tests/test_quality_policy.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import tomllib
 from copy import deepcopy
 from datetime import date
 from pathlib import Path
@@ -11,9 +10,12 @@ from scripts import check_quality_policy as quality
 
 
 def test_mutation_campaign_avoids_native_extension_reload_mode():
-    pyproject = tomllib.loads(Path("pyproject.toml").read_text())
+    pyproject = Path("pyproject.toml").read_text()
+    mutmut_config = pyproject.split("[tool.mutmut]", maxsplit=1)[1].split(
+        "\n[", maxsplit=1,
+    )[0]
 
-    assert pyproject["tool"]["mutmut"]["mutate_only_covered_lines"] is False
+    assert "mutate_only_covered_lines = false" in mutmut_config
 
 
 def _policy():