use psycopg3's transaction in `BaseDb.transaction`

There is a dedicated logic in psycopg3, so lets simply uses it.

swh/core/db/__init__.py

@@ -184,14 +184,9 @@ class BaseDb:
             a psycopg cursor
 
         """
-        with self.conn.cursor() as cur:
-            try:
+        with self.conn.transaction():
+            with self.conn.cursor() as cur:
                 yield cur
-                self.conn.commit()
-            except Exception:
-                if not self.conn.closed:
-                    self.conn.rollback()
-                raise
 
     def copy_to(
         self,