Replace Travis CI with GitHub Actions

[hugovk]

Travis CI has a new credit-based model for open source, making it harder to use. For example, we can see recent PRs don't have any Travis CI run: https://github.com/idank/bashlex/pull/59

Many projects have moved to GitHub Actions.

This PR moves CI to GitHub Actions and tests the same Python versions: 2.7 and 3.5-3.6.

It also adds Python 3.7-3.9, and testing on macOS and Windows as well as Linux.

And it also adds pip caching, to help reduce the strain on PyPI.

Here's an example build:

• https://github.com/hugovk/bashlex/runs/3662946695?check_suite_focus=true

[idank]

This looks good, can you please squash the commits? Also if you could change the commit message summary to this format:

subject: short message

[hugovk]

Done!

https://github.com/hugovk/bashlex/actions/runs/1265653981

[idank]

nit: can you lowercase the commit message? :)

[hugovk]

Sure, even lowercase the proper nouns?

[idank]

just wanted the first word to be lowercase, so up to you 🥇.

[hugovk]

Done!

[idank]

Thanks so much!