unittest: fix assertion errors on unittest reruns

This fixes unittest test reruns when using plugins like
pytest-rerunfailures.

The `instance` property uses AttributeError to check if the instance
needs to be initialized, so `del` is the correct way to clear it, not
setting to `None`.

Regressed in 8.2.2.

testing/plugins_integration/pytest_rerunfailures_integration.py

@@ -0,0 +1,11 @@
+import unittest
+
+
+class MyTestCase(unittest.TestCase):
+    first_time = True
+
+    def test_fail_the_first_time(self) -> None:
+        """Regression test for issue #12424."""
+        if self.first_time:
+            type(self).first_time = False
+            self.fail()