ci: migrate from Travis and AppVeyor to GitHub Actions

[eine]

Example run: https://github.com/1138-4EB/vunit/runs/258363743

Now that GitHub Actions (GHA) are enabled for the VUnit organization, this is work-in-progress to migrate from Travis and AppVeyor. ToDo:

• Add .github/workflows/push.yml, which is executed after each push or PR.
• Remove .travis.yml.
• Remove .appveyor.yml.
• Add key/secret and test deployment to gh-pages.
• Test deployment to PyPI with twine. @LarsAsplund, @kraigher, any experience with it?
• Add macOS job(s)?
• Coloured logs. See below.
• Merge VUnit/docker as .github/workflows/images.yml. (ci: add GHA workflow 'images.yml' #571)
• Drop support for Python 3.4, because it reached end-of-life (https://www.python.org/downloads/release/python-3410/) and it is not available in neither ubuntu-latest nor windows-latest environments. [virtual-environments-for-github-actions#supported-virtual-environments-and-hardware-resources] (7ec36ea)
• Avoid setting if: github.event_name == 'release' && github.event.action == 'created' in each step of job deploy. [Condition to run a job actions/starter-workflows#112]

---

Currently, these are the test matrices:

linux: [
          37-fmt,
          37-lint,
          27-unit,
          35-unit,
          37-unit,
          27-docs,
          37-docs,
],
docker (linux): [
  {do: acceptance,  ver: 27, img: mcode-2},
  {do: acceptance,  ver: 37, img: llvm},
  {do: vcomponents, ver: 37, img: mcode},
],
windows: [
          27-acceptance-ghdl,
          37-acceptance-ghdl,
          37-vcomponents-ghdl,
          37-lint,
          27-unit,
          35-unit,
          37-unit,
          27-docs,
          37-docs,
]

I wonder if we should add these jobs:

• GNU/Linux:
  • vcomponents with LLVM backend.
  • acceptance with mcode backend.
• Windows:
  • acceptance and vcomponents with LLVM backend.

There are 19 jobs already, and adding these would sum up to 23. When Python 2 is deprecated (scheduled for January 1, 2020), the number of jobs will be reduced to 13 (17).

[eine]

Regarding coloured logs, on the current master branch:

• GNU/Linux (Travis): all the jobs in docker containers have coloured logs, but jobs executed on the 'host' do not: https://travis-ci.org/VUnit/vunit/builds/583016616
• Windows (AppVeyor): none of the jobs has coloured logs: https://ci.appveyor.com/project/kraigher/vunit/builds/27300393

This PR: https://github.com/1138-4EB/vunit/commit/8f3483046913065d219007a46678198a5ec0fd9b/checks

• Jobs executed on the host VM with pytest (i.e. *-lint, *-unit, -Windows only- *acceptance*) require --color=yes to be added to tox.ini.
• Jobs executed in docker GNU/Linux containers (with pytest -*acceptance*-, or without it -*vcomponents*-), have coloured logs without further modifications.
• For jobs executed without pytest ( *vcomponents* on Windows) see Improve IS_WINDOWS_SYSTEM check #539.

Nonetheless, I am not happy with adding --color=yes to the tox.ini file. Colour support is automatically enabled on docker containers. Ideally, we should be able to start shells in GHA with the appropriate environment. It would allow us not to 'force' colours in each tool explicitly. See actions/starter-workflows#114 and tartley/colorama#214.

[eine]

Updates:

• This PR is now based on Remove Python 2.7 support #596; hence, we won't merge this until Python 2 is removed from master.
• Assuming that the next step will be to deprecate/remove Python 3.5 in order to support mypy, a commit in this PR changes it: 74ae885 Nevertheless, this commit can be split to a separate PR.
• The relevant content, where workflow 'push' is added, is contained in a single script: 40c1a48 Compared to the comments above, the proposal is not to replace Travis and AppVeyor, but to use the three services:
  • Travis for Python3.8 tests on GNU/Linux only, and for releasing/publishing to PyPI.
  • Appveyor for Python 3.8 tests on windows only.
  • GHA for all other GNU/Linux and Windows tests (3.5/3.6, 3.7, fmt, docs, etc), and for publishing the docs to vunit.github.io.

This approach is expected to be less disruptive than doing the replacement at once.

• The last commit is where .travis.yml and .appveyor are removed, and releasing is managed through twine in GHA: 15e5034 We can delay this until we finish deprecating and reorganizing the codebase.

Regarding colouring (#536 (comment)), the current solution is acceptable, since modifications to tox.ini are quite clean: https://github.com/VUnit/vunit/pull/536/files#diff-b91f3d5bd63fcd17221b267e851608e8

Note that the intermediate commit, where the three services are used, fixes colouring (tox and pytest) on Travis CI too.