ci: migrate from Travis CI to GitHub Actions

[umarcor]

Close #11.
Close #14.

This PR is based on #14.

• CI is migrated from Travis CI to GitHub Actions.
• The list of Python versions is extended to include 3.8 and 3.9.
• tox.ini is updated accordingly.

See https://github.com/umarcor/vtr-xml-utils/actions/runs/397817494.

/cc @acomodi

[litghost]

CI went red:

ERROR: ./setup.py does not have license information.
ERROR: ./tests/test_converting.py does not have license information.
ERROR: ./vtr_xml_utils/__main__.py does not have license information.
ERROR: ./vtr_xml_utils/convert.py does not have license information.

Same as test run.

[umarcor]

@litghost, yes, that's expected because #10 was not merged yet. See #14 (comment).