Fix for test failures with free-threaded Python

Look for changes in the number of allocated Vectors instead of whether 
there are any. For some reason, I'm getting Vectors already allocated 
when running pytest on the free-threaded version of Python. They are 
clearly referenced by something else that has run before these tests.

Author: jonathanhogg

tests/test_vm.py

@@ -1086,10 +1086,17 @@ def tearDownClass(cls):
     def setUp(self):
         gc.collect()
         gc.disable()
+        self._vector_count = 0
+        for obj in gc.get_objects(0):
+            if isinstance(obj, Vector):
+                self._vector_count += 1
 
     def tearDown(self):
+        vector_count = 0
         for obj in gc.get_objects(0):
-            self.assertNotIsInstance(obj, Vector, "Memory leak")
+            if isinstance(obj, Vector):
+                vector_count += 1
+        self.assertEqual(vector_count, self._vector_count, "Memory leak")
         gc.enable()
 
     def test_create(self):