Minor code improvement in `sage.doctest` #18459

Job	Run time
test-new	20m 31s
test-long (src/sage/[a-f]*)	48m 5s
test-long (src/sage/[g-o]*)	54m 18s
test-long (src/sage/[p-z]*)	48m 24s
test-long (src/doc src/sage_docbuild src/sage_setup)	26m 40s
coverage-report	26m 40s
	3h 44m 38s

Provide feedback