Add a test class inheriting unittest.TestCase to exemplify a bug

- This commit shows how pytest does not seem to retry the failing test
  case if it's inheriting from TestCase. Bug?

test/test_pytest_example.py

@@ -9,6 +9,7 @@
 import pytest
 # pylint:enable=import-error
 from flaky import flaky
+from unittest import TestCase
 
 
 @flaky
@@ -74,3 +75,16 @@ def test_flaky_thing_that_fails_then_succeeds(self):
         """
         self._threshold += 1
         assert self._threshold >= 1
+
+
+@flaky
+class TestExampleFlakyTestsThatsInheritingTestCase(TestCase):
+    _threshold = -1
+
+    def test_flaky_thing_that_fails_then_succeeds(self):
+        """
+        Flaky will run this test twice.
+        It will fail once and then succeed.
+        """
+        self._threshold += 1
+        assert self._threshold >= 1