bpo-42213: Remove redundant cyclic GC hack in sqlite3 (GH-26517)

The sqlite3 module now fully implements the GC protocol, so there's no
need for this workaround anymore.

- Add and use managed resource helper for connections using TESTFN
- Sort test imports
- Split open-tests into their own test case

Automerge-Triggered-By: GH:vstinner

Modules/_sqlite/connection.c

@@ -149,14 +149,6 @@ pysqlite_connection_init(pysqlite_Connection *self, PyObject *args,
         return -1;
     }
 
-    /* By default, the Cache class INCREFs the factory in its initializer, and
-     * decrefs it in its deallocator method. Since this would create a circular
-     * reference here, we're breaking it by decrementing self, and telling the
-     * cache class to not decref the factory (self) in its deallocator.
-     */
-    self->statement_cache->decref_factory = 0;
-    Py_DECREF(self);
-
     self->detect_types = detect_types;
     self->timeout = timeout;
     (void)sqlite3_busy_timeout(self->db, (int)(timeout*1000));