fix: satisfy mypy 2.3.0 [deprecated] and [unreachable] checks

- tox_run.py: os.system is soft-deprecated, use subprocess.call
- test_monkeypatch.py: inspect.isclass is now typed as TypeIs[type],
  narrowing makes the post-context assert look unreachable

Author: Pierre-Sassoulas

testing/freeze/tox_run.py

@@ -8,9 +8,10 @@
 
 if __name__ == "__main__":
     import os
+    import subprocess
     import sys
 
     executable = os.path.join(os.getcwd(), "dist", "runtests_script", "runtests_script")
     if sys.platform.startswith("win"):
         executable += ".exe"
-    sys.exit(os.system(f"{executable} tests"))
+    sys.exit(subprocess.call([executable, "tests"]))

testing/test_monkeypatch.py

@@ -436,7 +436,7 @@ def test_context() -> None:
     with monkeypatch.context() as m:
         m.setattr(functools, "partial", 3)
         assert not inspect.isclass(functools.partial)
-    assert inspect.isclass(functools.partial)
+    assert inspect.isclass(functools.partial)  # type: ignore[unreachable]
 
 
 def test_context_classmethod() -> None: