Covariance spectrum fixes #2041

Job	Run time
check_commit	4s
ci-tests (Black test, ubuntu-latest, 3.10, black, false)	16s
ci-tests (Py3.8 with old Astropy version, ubuntu-latest, 3.8, py38-test-astropy4-cov, false)	2m 34s
ci-tests (Linux, Py3.10 with coverage, ubuntu-latest, 3.10, py310-test-cov, false)	2m 18s
ci-tests (Windows, Py3.11 with coverage, windows-latest, 3.11, py311-test-cov, false)	3m 28s
ci-tests (Mac OS M1, Py3.11, macos-14, 3.11, py311-test, false)	1m 50s
ci-tests (Mac OS, Py3.11, macos-latest, 3.11, py311-test, false)	2m 16s
ci-tests (Linux, Py3.11 all dependencies and coverage, ubuntu-latest, 3.11, py311-test-alldeps-co...	4m 3s
ci-tests (Linux, Py3.12 basic dependencies and coverage, ubuntu-latest, 3.12, py312-test-cov, tru...	2m 19s
ci-tests (Linux, Py3.11 with dev versions of dependencies, ubuntu-latest, 3.11, py311-test-devdep...	2m 33s
ci-tests (Slow tests on Linux, Py3.11, all deps and coverage, ubuntu-latest, 3.11, py311-test-all...	10m 42s
ci-tests (Slow tests on Linux, Py3.11, basic deps and coverage, ubuntu-latest, 3.11, py311-test-c...	4m 29s
	36m 52s

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