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/cache.h

@@ -52,10 +52,6 @@ typedef struct
 
     pysqlite_Node* first;
     pysqlite_Node* last;
-
-    /* if set, decrement the factory function when the Cache is deallocated.
-     * this is almost always desirable, but not in the pysqlite context */
-    int decref_factory;
 } pysqlite_Cache;
 
 extern PyTypeObject *pysqlite_NodeType;