Fix fragile imports

[neutrinoceros]

PR Summary

For a while there was a note in pyproject.toml that said isort couldn't be applied to yt/init.py because it would create circular imports. Turns out there are only a handful of fragilities to fix in that regard, and I'm doing it here.

[neutrinoceros]

Indeed the one test that failed was the one defining a sample plugin file: it broke because np wasn't defined anywhere else.
I went ahead and fixed it but this does mean that this removal is able to cause breakage downstream...

[matthewturk]

I think able, but not likely.

…

On Mon, Jan 4, 2021 at 10:55 AM Clément Robert ***@***.***> wrote: Indeed the one test that failed was the one defining a sample plugin file: it broke because np wasn't defined anywhere else. I went ahead and fixed it but this *does* mean that this removal is able to cause breakage downstream... — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#3016 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAVXOYH4IUU2VRW22C6XT3SYHXGPANCNFSM4VSAGSWA> .

[neutrinoceros]

Well merge away if you're up for it, but I'm adding the "backward incompatible" label for the release manager because the natural error message is really not helpful:

# from the test
...
yt.utilities.exceptions.YTFieldNotFound: Could not find field ('unknown', 'random') in UniformGridData.

users should be warned that they need to import numpy themselves from the plugin file if they intend to use it there.