Test with GitHub Actions

[hugovk]

Includes #64.

Travis CI has a new pricing model making it harder to test open source projects, and it's also stopped running.

This PR add testing on GitHub Actions.

Pros

• 20 parallel jobs, not 5 like Travis
• Can test on Windows and macOS, not just Ubuntu
• Very flexible, lots of extra actions (aka add-ons) available
• The new popular CI for Python projects
• No confusing pricing model

Cons

• Does not have ppc64le architecture
• Does not have EOL Python 3.4 (I would recommend dropping EOL 2.7 and 3.4-3.6)

If ppc64le isn't important, I'd recommend deleting .travis.yml outright.

Demo

https://github.com/hugovk/httmock/actions/runs/1677534178