Fix numba call for deprecation of nopython=True

[aarmey]

Fixes #1048 for umap.plot.

I think this line was missed during the changes in v0.5.4.

[lmcinnes]

This is actually a nopython=False case; I think in practice it probably needs to be updated to use the python object context handling that has been added to numba. I'll try to get around to that in due course.

[pep8speaks]

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

• In the file umap/plot.py:

Line 209:89: E501 line too long (92 > 88 characters)

[aarmey]

@lmcinnes you might want to avoid the additional complexity, but here is a version that can be fully compiled by Numba.