chore: Register 'serial' pytest marker in mlops tox.ini

The Lake Formation integ tests use @pytest.mark.serial, but the marker
was never registered, producing PytestUnknownMarkWarning noise on every
run. Register it in the [pytest] markers section.

Author: lucasjia-aws

sagemaker-mlops/tox.ini

@@ -64,6 +64,7 @@ markers =
     release
     image_uris_unit_test
     timeout: mark a test as a timeout.
+    serial: mark a test that must not run concurrently with others sharing the same resources.
 
 [testenv]
 setenv =