Remove astropy-helpers!

[larrybradley]

This PR is a followup to #914, simplifying the package setup by completely removing astropy-helpers. 🚀 Worked on during the pyastro hack day with help from @astrofrog.

This PR includes a minimal tox configuration for 'build_docs' and 'test' (both work fine).

Some notes:

• I discovered that setuptools_scm cannot be used with packages that still rely on astropy-helpers to build extensions. The astropy-helpers extension builder looks for the version.py module generated by helpers.

• I put the Cython extension builds in a separate module called setup_extensions.py. As discussed with @astrofrog, it would great if a small package could handle building extensions.

• I added dummy test and build_docs setup commands in setup_commands.py that print a message to the user that those commands no longer work. Again, it would be nice for that boilerplate to go into a helper package.

• Also discussed with @astrofrog -- having a pytest plugin to handle the -P option is also needed!

@astrofrog Please take a look and try it out and let me know if I missed anything.

This PR is not ready to be merged.

[astrofrog]

@larrybradley - I'm happy to take a look at this to see if anything can be simplified further once you rebase :)

[larrybradley]

@astrofrog It's rebased.

[pep8speaks]

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

• In the file photutils/conftest.py:

Line 9:80: E501 line too long (84 > 79 characters)
Line 37:80: E501 line too long (80 > 79 characters)

Comment last updated at 2020-05-05 17:21:49 UTC

[astrofrog]

Is the removal of 32-bit testing deliberate?

[larrybradley]

No, definitely not. I'm just playing with CircleCI config. I had assumed astropy/image-tests-py37-mpl311:1.10 was 32-bit, but included mpl.

I changed the 32-bit tests to include alldeps, but the issue I'm having is mpl cannot be installed in the quay.io/pypa/manylinux1_i686 image because of a missing freetype library:
https://app.circleci.com/pipelines/github/astropy/photutils/14/workflows/f4936108-6c07-4455-8001-e1d9087499fe/jobs/1149

Unless you know how to fix this, then I'm going revert to running the 32-bit tests only with the required dependencies (which does not include mpl).

[larrybradley]

Does the rest of this PR look OK? Everything is ready to be merged, except for this CircleCI issue.

[larrybradley]

There isn't a mpl wheel for 32-bit linux, thus CircleCI is compiling from source.

[codecov]

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[larrybradley]

These changes are based on astropy core and the package template, so I don't think they are controversial. I'm going to merge this PR now because it will fix the failing travis-ci and RTD builds.