fix: close session once, outside of for-loop

Signed-off-by: F.N. Claessen <felix@seita.nl>

flexmeasures/data/migrations/versions/ad98460751d9_remove_obsolete_tables.py

@@ -36,12 +36,12 @@ def upgrade():
             result = db.session.execute(
                 sa.text(f"SELECT EXISTS (SELECT 1 FROM {table});")
             ).scalar_one_or_none()
-            db.session.close()  # https://stackoverflow.com/a/26346280/13775459
             if result:
                 tables_with_data += [table]
         except ProgrammingError as exception:
             print(f"Table {table} not found, continuing...")
             print(exception)
+    db.session.close()  # https://stackoverflow.com/a/26346280/13775459
 
     if tables_with_data:
         click.confirm(