Tests

Fixed `mixins.startup.serve` UnboundLocalError. #634

Job	Run time
Linux / Python 3.8 / tox -e security	13s
Linux / Python 3.9 / tox -e security	1s
Linux / Python 3.10 / tox -e security	29s
Linux / Python 3.11 / tox -e security	31s
Linux / Python 3.10 / tox -e lint	39s
Linux / Python 3.8 / tox -e type-checking	10s
Linux / Python 3.9 / tox -e type-checking	1s
Linux / Python 3.10 / tox -e type-checking	43s
Linux / Python 3.11 / tox -e type-checking	43s
Linux / Python 3.8 / tox -e py38	43s
Linux / Python 3.8 / tox -e py38-no-ext	43s
Linux / Python 3.9 / tox -e py39	4m 7s
Linux / Python 3.9 / tox -e py39-no-ext	1s
Linux / Python 3.10 / tox -e py310	42s
Linux / Python 3.10 / tox -e py310-no-ext	44s
Linux / Python 3.11 / tox -e py311	41s
Linux / Python 3.12 / tox -e py312	44s
Linux / Python 3.11 / tox -e py311-no-ext	43s
Windows / Python 3.8 / tox -e py38-no-ext	40s
Windows / Python 3.9 / tox -e py39-no-ext	21s
Windows / Python 3.10 / tox -e py310-no-ext	41s
Windows / Python 3.11 / tox -e py310-no-ext	37s
	14m 57s

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