CI/CD Overhaul

[MichaelStott]

To help better maintain this project, the CI/CD pipeline needs to be revisited to provide better coverage and testing.

• Move Travis CI/CD to Github Actions.
• PEP16 style checks using Black.
• Automate building KivMob demo apps against multiple Android versions during testing. This buildozer Github action might help: https://github.com/ArtemSBulgakov/buildozer-action
• If possible, test AdMob functionality against a mock class.