chore: Register 'serial' pytest marker in mlops tox.ini #1128

Job	Run time
collab-check	5s
wait-for-approval	4s
detect-changes	13s
integ-tests (sagemaker-serve)	43m 37s
unit-tests (sagemaker-serve)	37m 2s
unit-tests (sagemaker-mlops)	29m 55s
integ-tests (sagemaker-mlops)	1h 36m 19s
integ-tests-us-east-1	42m 30s
codestyle-doc-tests (sagemaker-mlops)	6m 55s
codestyle-doc-tests (sagemaker-serve)	5m 57s
	4h 22m 37s

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