Enable GitHub Actions on CI

[krassowski]

• [x] Replace Travis CI (which is no longer an option for open-source projects) with GitHub Actions for testing
• [x] Improve the CI scripts to use reproducible conda environment with fast mamba installation and to fail early
• [x] Add support for using R installation separate from the system installation (and use it on CI)
• [x] Use non-local MySQL on CI
• [x] Add flake8 to ensure acceptable Python code quality
• [x] Fix all issues highlighted by flake8