Features: 1) Add match-case logic for selecting gateway in process_transaction_changes.

Fixes: None.

Extra: 1) Minor adjustment to default `AbstractGateway` instantiation.

payments/signals.py

@@ -17,7 +17,13 @@ def create_balance_on_user_creation_signal(instance, created, **_kwargs):
 def process_transaction_changes(instance, created, **_kwargs):
     if created:
         try:
-            gateway = AbstractGateway()
+            match instance.process.get("gateway", "default"):
+                case "gateway":
+                    gateway = AbstractGateway
+                case "default":
+                    gateway = AbstractGateway
+                case _:
+                    gateway = AbstractGateway()
             gateway.process_transaction(instance)
         except Exception as e:  # noqa:
             instance.process = {"status": "NOGATEWAY", "error": str(e)}