Add GitHub Actions CI workflow

[elliotwutingfeng]

Closes #43.

• Added GitHub Actions CI workflow for Python versions 2.7 and 3.8-3.12.
• Some bare exceptions for Python 2/3 checks were replaced with more specific methods like sys.version_info checks or ImportError.

The ubuntu container was needed for Python 2.7 as none of the current runners support Python 2.7 directly anymore.