clean up imports of `jit_ext.py` #5416

Job	Run time
pytester (ubuntu-22.04, 3.10, latest, core)	12m 6s
pytester (ubuntu-22.04, 3.10, latest, ops)	30m 1s
pytester (ubuntu-22.04, 3.10, nightly, core)	12m 10s
pytester (ubuntu-22.04, 3.10, nightly, ops)	29m 59s
pytester (macOS-14, 3.10, latest, core)	8m 57s
pytester (macOS-14, 3.10, latest, ops)	23m 39s
pytester (macOS-14, 3.10, nightly, core)	8m 39s
pytester (macOS-14, 3.10, nightly, ops)	23m 21s
pytester (windows-2022, 3.10, latest, core)	25m 7s
pytester (windows-2022, 3.10, nightly, core)	26m 25s
pytester (ubuntu-22.04, 3.11, latest)	24m 18s
pytester (ubuntu-22.04, 3.12, latest)	28m 1s
testing-guardian	1s
	4h 12m 44s

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