WIP: First attempt at GitHub Actions integration for CI

[bocklund]

Checklist

• The documentation examples have been regenerated if the Jupyter notebooks in the examples/ have changed. To regenerate the documentation examples, run jupyter nbconvert --to rst --output-dir=docs/examples examples/*.ipynb from the top level directory)
• If any dependencies have changed, the changes are reflected in the
  • setup.py
  • .travis.yml (deleted)
  • appveyor.yml (deleted)
  • conda_recipe/meta.yaml (deleted)
  • environment-dev.yml (added)

[bocklund]

This is a work in progress PR. The goals are

• tests pass for Windows
• tests pass for Mac
• test pass for Linux
• we have a second CI script that will depend on the build matrix (above platforms * Python versions) passing that does the documentation deployment
• coveralls code coverage integration works
• bonus (a relatively more) unified and sane way of tracking dependencies

Other TODO items:

• Can we drop coveralls<5 pin now?
• Make sure to cleanup (delete?) environment.yml/environment-test.yml depending on whether or not they're actually used
• cleanup old travis, appveyor and ci/ files.

[bocklund]

Woohoo! All 3 platforms are passing with python 3.8 on f4cd558 🎉

[bocklund]

Once everything passes, we have accomplished all the original goals of this PR.

The only things not at feature parity are:

• Coverage doesn't report for PRs that originate from branches outside the pycalphad repo (i.e. forks)
• Documentation only deploys to the latest directory of the website branch (whenever a commit shows up on develop). We aren't deploying a separate directory for every tag like we were before. It's possible to do this, but either requires duplicating the docs.yaml and making some minor changes, or adding parsing of the github.ref variable and applying some logic to deploy to "latest" if not tagged and the right tag if tagged.

Ultimately, what's here in GitHub Actions is just a starting point to organically grow the infrastructure from. Some outstanding things we could do are:

• Combine the tests and docs into one workflow where the Docs job needs: Tests and won't run until all the test succeed
• Deploy tagged commits to PyPI

[bocklund]

This is ready for one last look @richardotis