Add a continuous integration worflow

[tgdif]

Feature Request

The proposal is to setup a GitHub workflow to run checks on every commit. For open source repositories this is free.

Motivation

This would be a quick way to check that a commit passes linting and unit tests (possibly other checks too) and could speed up reviews and identify issues.

Design Proposal

Implementation Plan

Add the necessary file to the .github/workflows directory. This has been done on a fork here https://github.com/tgdif/pyoneer/tree/continuous_integration. Note that the commit is failing one check but that is due to a separate issue with one of the existing unit tests.

Alternatives Considered

There are a variety of alternatives (Travis, CircleCI, Azure Pipelines, etc.) though this looks to be the simplest way and does not require adding a separate app.

[tgdif]

Should I also upgrade some of the dependent packages in Pipfile and setup.py? These affect the environments used when running the workflow. Since tensorflow and tensorflow-probability now support TF version 2.0.0 by default on PyPI their versions can be bumped up.