Move hips package from Python 3.5 to 3.6

[cdeil]

Like I've mentioned in a note at https://hips.readthedocs.io/en/latest/installation.html I would suggest to only support Python 3.6+ for the hips package, i.e. to drop Python 3.5 support.

@tboch or anyone - if you think it's a bad idea and we should support Python 3.5, please comment. This is more of a "nice to be able to use the latest features as a developer", there is nothing in Python 3.6 that we absolutely need, we can support 3.5.

For now, I'm holding off on this change because:

• healpy packages for Python 3.6 are missing on conda-forge, i.e. we can't switch travis-ci to Python 3.6 yet.
• readthedocs doesn't support Python 3.6 yet (at least they didn't 2 weeks ago, should be resolved soon).

So this is a reminder issue for myself, for now assigned to the v0.2 milestone.

[cdeil]

I've attached a commit here that uses f-strings, a feature added in Python 3.6, i.e. drops Python 3.5 compatibility. travis-ci was already changed to run on Python 3.6, so I expect tests to pass.

[coveralls]

Coverage remained the same at 80.426% when pulling 0bacc94c279de11526845dd2d8bbcd73385aedad on cdeil:py36 into f08fa2a28edf57f035a673a5e131888121b6e9ea on hipspy:master.

[cdeil]

Looks like the change Python 3.5 to 3.6 on ReadtheDocs via this commit in master worked:

• https://github.com/hipspy/hips/commit/00e9dea625a6d9bf19fa21a13a606531c6da7279
• https://readthedocs.org/projects/hips/builds/5629248/

[cdeil]

I've cherry-picked 4baa6434a87ab754167be1b32006cff6a1070720 into master. Closing this issue now.

[coveralls]

Coverage remained the same at 80.426% when pulling 0bacc94c279de11526845dd2d8bbcd73385aedad on cdeil:py36 into f08fa2a28edf57f035a673a5e131888121b6e9ea on hipspy:master.