migrate from Travis to GitHub Actions and stop testing Python 2

[StrikerRUS]

Quite big CI refactoring. There are two main changes in this PR:

• Python 2 reached its EOL 1/1/2020. It was more than year ago, so there is no need to support it anymore.
• Why do we need to migrate from Travis? Refer to: [RFC] Travis adding much stricter limits on free users microsoft/LightGBM#3519. GitHub Actions is better than Travis in terms of offered capabilities in free tier: https://docs.github.com/en/actions/reference/usage-limits-billing-and-administration#usage-limits. Only one frustrating thing that I noticed during more than 6 months of usage is that you cannot re-run a separate job, you should rerun all of them.

Also, during refactoring I containerized everything for Linux jobs. It greatly reduces test time and complexity of dependency management.

R-package tests now use the latest available version of R. It is 4.0.3 right now. This is great, I think!

scikit-learn version should be restricted for now because starting from 0.24.0 our integration test fails due to some values in sample-weight array are zeros. I'm going to address this issue in a follow-up PR.

[StrikerRUS]

Oh, forgot to mention that I'm also incrementing macOS version in Python wheel filename according to #309 (comment).

[StrikerRUS]

... And I haven't transferred creating i686 Linux Python wheels. I don't think that they are in demand nowadays.

[mlampros]

good work, thanks (especially for the r part of the code)

[fukatani]

@StrikerRUS
Thank you for PR!

Appveyor was failed.
Could you check this?

[StrikerRUS]

Thanks a lot guys for reviews!

@fukatani

Appveyor was failed.
Could you check this?

Ah yeah, sure! Seems that filename of installer inside zip archive has been changed. I just pushed updates in 4d28d9b. Hope it will make all checks green!

[StrikerRUS]

@fukatani All checks are green now! 👍
Could you please leave your review?

[fukatani]

LGTM
Thank you for your contribution