Migrate codebase to Python 3.8 or later

[alex-l-kong]

Describe the bug

With Python 3.6 having reached EOL on December 23, 2021, our codebase is due for an upgrade. Several libraries have stopped supporting Python 3.6 or will stop soon. Additionally, PyPI now requires a codebase of at least 3.7; trying to deploy with python: 3.6 in .travis.yml produces this error:

To Reproduce

For PyPI, try to create a tagged commit, which includes making a new release.

Expected behavior

Deployment to PyPI should be successful on a tagged commit or new release.

Additionally, our code should be compatible with at least Python 3.8.

Addressing the Issue

When addressing the issue, please ensure that the following are addressed:

• All tests run in a Python 3.8 or later environment
• The Dockerfile now specifies Python 3.8 or later and successfully builds
• .travis.yml runs the tests on the Travis CI and deploys to PyPI/Docker Hub on Python 3.8 or later

Additional Notes

Because Python 3.7 will reach EOL on June 27, 2023, we want to skip this version to avoid another migration the following year. Python 3.8 has an EOL of October 14, 2024; Python 3.9 has an EOL of October 5, 2025. These would be better options.