CI setup on GitHub Actions, nbdev and a basic test

[ntoxeg]

So, this is a CI setup that I proposed in #25.

• nbdev is setup and used in CI to do sanity checks on notebooks (whether they are clean etc.)
  • settings.ini is used for general info about the package, setup.py reads dependencies from requirements.txt. Overall these files won’t require much changing from now on.
• I chose pytest for testing because it is very popular, easy to setup and it can even run tests from other frameworks (like nose).
• Unfortunately, I couldn’t get the MineRL code to run - probably because GitHub expects the pipeline to run as root and it can’t find a display for the agent.
• Added a simple workflow for tagging stale issues / PRs.
• Added a trivial test that checks if some OpenCog libraries can be imported.

Maybe I will figure out how to run Gym / MineRL code - like getting to run the workflow as a normal user or some other way. Thus, for now checking if notebooks execute is disabled.

[ntoxeg]

Rebased

[ntoxeg]

@ngeiswei review please.

[ntoxeg]

Since I’m adding formatting changes on top of the CI setup, I’ve included the devcontainer setup as well - it doesn’t change anything for anyone not using it but can be nice for new users (and I could finally stop awkwardly throwing these file around without adding them to the repository).

[ntoxeg]

As for the Makefile, it’s not necessary - it just comes from the nbdev template. For now most of its functionality is not relevant (like generating documentation), so I can remove it if you wish. If black is to be used though you can now use it to conveniently generate code from notebooks and format it at the same time.

As for the formatting, it’s your call @ngeiswei - I can revert it and just fix the missing newline manually (which will have to be done every time code from notebooks is generated until the Issue I posted is resolved in nbdev). One important change now is that everyone will have to format the code with black, so if you don’t like the idea of enforcing that I can drop it.

[ngeiswei]

All seems to function, I'm merging. Thanks again @ntoxeg !