chore: add sitecustomize for plugin loading (#5283) #250

Job	Run time
lint / files	1m 24s
test / Fast tests (ubuntu-24.04)	1m 50s
test / Minimum dependencies (all tests)	15m 45s
test / Slow tests (ubuntu-24.04, 3.12)	14m 26s
	33m 25s

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