Skip slow tests unless --run-slow is specified.

[mkelley]

~pytest --run-slow~

~OR~

~python3 setup.py test --args=--run-slow~

Update: use pytest -m 'not slow'

[mkelley]

To do: run test suite on v0.3 release and mark slowest tests as slow.

[pep8speaks]

Hello @mkelley! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers:

Comment last updated at 2022-02-07 14:17:14 UTC

[mkelley]

TBD: switch to pytest-astropy v0.10, and we can use @pytest.mark.slow and --run-slow or -m slow.

[mkelley]

Closing in favor of #385