Dependency cycle with specutils: python-ndcube -> python-specutils -> python-ndcube

[Hellseher]

Provide a general description of the issue or problem.

Hi,

During update Guix's astro soft stack, I've noticed that ndcube@2.2.1 and higher
started depends on specutils for running tests, which introduced the cycle

python-ndcube -> python-specutils -> python-ndcube

To resolve it during unit tests I've applied some modifications:

• -k not test_rebin_specutils
• Remove from specutils import Spectrum1D from ndcube/tests/test_ndcube.py

Thanks,
Oleg