Modernization galore

[pllim]

Please use SQUASH AND MERGE button!

• Use tox for CI
• Add flake8 check and settings; as a result also have to fix some PEP 8 complains
• Add test header for our sanity
• Move setup.cfg and coveragerc stuff to pyproject.toml
• Move benchmark out of unit tests area (it is not something pytest can/should run)
• Consolidate test helper functions into a proper helpers.py
• Replace relative with absolute imports, fix #242
• Use OpenAstronomy workflow for CI and publishing, fix #239
• Enable CI testing in different OSes
• Enable both compulsory and optional wheel building (but not uploading) for PRs
• Add installation instructions, fix #204
• Fix image not showing on RTD but it also has to show on GitHub README, fix #69
• Work around #244
• Work around #245

After merge:

• ~Open follow up issue about CI not able to see math_util C-ext (NUMPY_EXPERIMENTAL_DTYPE_API) and the xfail. Maybe tox is preventing the CI to see the .so file but I don't know how to fix. synphot works just fine with tox (though not OpenAstronomy) and astropy also works fine (with OpenAstronomy).~ (fixed!)
• ~Open issues about aarch64 wheels unable to be built (takes too long).~ #247
• ~Update branch protection rules.~ (done!)
• Someone with admin access to PyPI for this package needs to generate a token and add it as PYPI_TOKEN repo secret here. #249
• In the next release, merely a tag would generate a release, so don't tag for fun!
• In the next release, if successful, users will be able to get all sorts of wheels from PyPI and have to rely less on conda.

[codecov[bot]]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
spherical_geometry/__init__.py	100.00% <100.00%> (ø)
spherical_geometry/graph.py	93.19% <100.00%> (+0.03%)	:arrow_up:
spherical_geometry/polygon.py	82.37% <100.00%> (-0.05%)	:arrow_down:
spherical_geometry/vector.py	84.48% <ø> (+2.81%)	:arrow_up:
spherical_geometry/great_circle_arc.py	39.53% <12.50%> (-1.69%)	:arrow_down:

... and 1 file with indirect coverage changes

:loudspeaker: Thoughts on this report? Let us know!.

[pllim]

Despite the scary diff, users won't actually see anything different. All they will get are new wheels after next release. So no need for change log.

[pllim]

This is in a good state now, so I am going to YOLO merge it. If you run into problems, will be happy to come back to help. Enjoy your wheels!