Force GC during weakref tests

Required to run the tests under PyPy with no refcount. See
https://github.com/mvantellingen/psycopg2-ctypes/pull/15#issuecomment-3274618

tests/test_async.py

@@ -435,9 +435,11 @@ class AsyncTests(unittest.TestCase):
         self.assert_(self.conn.notices)
 
     def test_async_cursor_gone(self):
+        import gc
         cur = self.conn.cursor()
         cur.execute("select 42;");
         del cur
+        gc.collect()
         self.assertRaises(psycopg2.InterfaceError, self.wait, self.conn)
 
         # The connection is still usable

tests/test_connection.py

@@ -158,10 +158,12 @@ class ConnectionTests(unittest.TestCase):
 
     def test_weakref(self):
         from weakref import ref
+        import gc
         conn = psycopg2.connect(dsn)
         w = ref(conn)
         conn.close()
         del conn
+        gc.collect()
         self.assert_(w() is None)