Fix RGB dtype issue

[adl1995]

@cdeil As discussed on Slack, I have updated the dtype so that RGB drawing works fine.

[cdeil]

Thanks!

One small inline comment.

Can you paste a screenshot you get with this change for the example from the high-level docs, please? https://hips.readthedocs.io/en/latest/getting_started.html#make-a-color-sky-image

[cdeil]

Always add a one-line docstring mentioning what it is, what object type is returned, and also add a type annotation.

[coveralls]

Coverage increased (+0.02%) to 96.535% when pulling 9f0f797 on adl1995:jpg_tile_draw_issue into 2ab7f7a on hipspy:master.

[adl1995]

I just updated the docstring. Here is the output image I'm getting:

[coveralls]

Coverage increased (+0.03%) to 96.543% when pulling 6139a0b on adl1995:jpg_tile_draw_issue into 2ab7f7a on hipspy:master.

[cdeil]

@adl1995 - You must have been looking at an outdated image on the docs page.
It's all black now: https://hips.readthedocs.io/en/latest//plot_fits.png
I'll make a commit in master shortly to fix.

[cdeil]

Follow-up commits in master: 9b52a7a and 60a5d95

[cdeil]

I had to wipe RTD and trigger a completely fresh build, for the plot scripts from the docs build to re-run.
It worked https://readthedocs.org/projects/hips/builds/5698160/ and now the images on
http://hips.readthedocs.io/en/latest/getting_started.html are correct, except for the last remaining drawing issue #74 .