Migrate from Travis CI to GitHub Actions

[albertosottile]

In this PR, the CI flow that was on Travis CI is migrated to GitHub Actions. Some minor changes to the build chain and/or to the codebase were also applied, which are further detailed platform by platform hereafter.

• Windows/AppVeyor: no changes for now. As Actions also support Windows, we might migrate this part in the future.
• AppImage: no changes needed to the workflow.
• Debian: almost no changes needed, a call to apt-get update was added in the test script.
• Snapcraft
  • all the snaps built did not work in field tests. Hence, we decided to stop building snaps in our CI/CD. It remains to be seen what we will do with the version that is currently on the Snap store.
  • the snapcraft.yaml recipe was completely rewritten to use core18 as base snap, Python 3.6, and PySide2. After this change, we will not have a released platform based on Qt 4 anymore.
  • this could also allow us to increase the minimum Python version for Syncplay to 3.6. On paper, we still support >=3.4, hence I reverted in this PR the f-string accidentally introduced in Console improvements #327. However, python_mpv_jsonipc already requires 3.6.
• macOS
  • the build mechanism was completely changed since Travis. Before, we used Homebrew to install Python, Qt, and PySide2. Now we use the version of Python provided by the build image and we install PySide2 from the standalone wheels (which include an embedded copy of Qt).
  • Pros: we do not depend on Homebrew anymore (and on when they upgrade their formulas/bottles). We can build on macOS 10.15 (the image provided by GitHub Actions) a package that works also on older systems (tested down to 10.13).
  • Cons: a few bugs on PySide2 require some manual steps that make the build flow brittle (see here and here). An extra symlink creation is required, it Is unclear if this is a bug on py2app or a misconfiguration of the macOS image. EDIT: I created an issue on py2app.
  • Known issues: for the moment, we cannot use versions newer than Python 3.7/PySide2 5.13.1 due to a limitation of py2app (see here).

A few general comments about the two CI platforms:

• Travis: the .travis.yml file was deleted. The helper folder travis is now named ci. The build status badge in README now points to GitHub Actions.
• GitHub Actions:
  • artifacts are now stored directly on GitHub (e.g. here) instead of Bintray so, no extra authentication is needed. The Bintray helper files were deleted. AppVeyor still deploys on BIntray but does not require these helper files.
  • artifacts are zipped automatically before being downloaded from the UI. This is a known limitation of GitHub Actions that is planned to be removed.

[daniel-123]

I don't see any blocking problems with this PR.

[Et0h]

This looks good to me.