move from travis ci to github actions

[jneu]

This PR removed the Travis CI build configuration and adds a Github Action workflow. In addition:

1. The Python version is bumped from 3.6 to 3.7 as 3.6 is no longer supported.
2. The step building docs is commented out as additional development is required to get it to work.
3. The pypi upload credentials are removed as this is not compliant with pypi uploads from spacetelescope org repos.