Resolve incompatibility between `imp` and the `DeferredImportCallbackFinder` #5952

Job	Run time
lint/style-and-typos	48s
linux/3.9/bare-env	0s
${{ matrix.TARGET }}/${{ matrix.python }}${{ matrix.other }}	0s
process-coverage-linux	28s
process-coverage-osx	31s
process-coverage-win	45s
	2m 32s

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