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.c

@@ -97,9 +97,6 @@ pysqlite_cache_init(pysqlite_Cache *self, PyObject *args, PyObject *kwargs)
     }
 
     self->factory = Py_NewRef(factory);
-
-    self->decref_factory = 1;
-
     return 0;
 }
 
@@ -108,9 +105,7 @@ cache_traverse(pysqlite_Cache *self, visitproc visit, void *arg)
 {
     Py_VISIT(Py_TYPE(self));
     Py_VISIT(self->mapping);
-    if (self->decref_factory) {
-        Py_VISIT(self->factory);
-    }
+    Py_VISIT(self->factory);
 
     pysqlite_Node *node = self->first;
     while (node) {
@@ -124,9 +119,7 @@ static int
 cache_clear(pysqlite_Cache *self)
 {
     Py_CLEAR(self->mapping);
-    if (self->decref_factory) {
-        Py_CLEAR(self->factory);
-    }
+    Py_CLEAR(self->factory);
 
     /* iterate over all nodes and deallocate them */
     pysqlite_Node *node = self->first;

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;

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));