Tests

Fixed `mixins.startup.serve` UnboundLocalError. #633

Job	Run time
Linux / Python 3.8 / tox -e security	26s
Linux / Python 3.9 / tox -e security	1s
Linux / Python 3.10 / tox -e security	30s
Linux / Python 3.11 / tox -e security	30s
Linux / Python 3.10 / tox -e lint	36s
Linux / Python 3.8 / tox -e type-checking	38s
Linux / Python 3.9 / tox -e type-checking	1s
Linux / Python 3.10 / tox -e type-checking	38s
Linux / Python 3.11 / tox -e type-checking	41s
Linux / Python 3.8 / tox -e py38	39s
Linux / Python 3.8 / tox -e py38-no-ext	39s
Linux / Python 3.9 / tox -e py39	37s
Linux / Python 3.9 / tox -e py39-no-ext	1s
Linux / Python 3.10 / tox -e py310	41s
Linux / Python 3.10 / tox -e py310-no-ext	3m 39s
Linux / Python 3.11 / tox -e py311	40s
Linux / Python 3.12 / tox -e py312	39s
Linux / Python 3.11 / tox -e py311-no-ext	41s
Windows / Python 3.8 / tox -e py38-no-ext	36s
Windows / Python 3.9 / tox -e py39-no-ext	1s
Windows / Python 3.10 / tox -e py310-no-ext	38s
Windows / Python 3.11 / tox -e py310-no-ext	39s
	14m 11s

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