Migrate from travis to GitHub actions & add mypy to the build

[fuhrysteve]

Hi @cgoldberg!

Thought I'd stop by and offer some updates to the build. This includes:

• Continued python 2.7 support
• python 2.7 compatible typing
• adds mypy in the build
• adds a code coverage report to the build which reports on which lines are covered and which are not. Helpful little report if you want to see if new or modified code has actual test coverage
• migrates from travis to github actions (travis has been problematic for a lot of maintainers lately)
• Updates python 3 supported versions to currently supported versions (3.6-3.9)
• adds codeql security scanning to the build (checks for common CVE vulnerabilities and will report on them in the security tab on GitHub)

Please also note that this PR will not trigger a build, because I'm not an owner of this repository. If you want to see evidence of a passing build, please see this PR on my own fork: fuhrysteve#1