Refactor project

# 💡 Summary #

This project should be refactored for multiple reasons.

## Motivation and context ##

An AWS Lambda just needs to be a single deployable artifact that only needs to support a single Lambda runtime. Given that premise we can pare down the Docker/[pipenv] configuration to target a single version of Python (corresponding with a supported AWS Lambda Python runtime). This should also allow us to integrate testing of the Python code much more easily.

## Implementation notes ##

- Reduce the [pipenv] configurations down to a single configuration targeting a single Python version.
- Reorganize the directory structure accordingly.
- Implement support for Python code testing (through [pytest]).
- Update the GitHub Actions workflows accordingly.

## Acceptance criteria ##

- [ ] There is only a single deployment artifact created.
- [ ] Python code testing is integrated.

[pipenv]: https://github.com/pypa/pipenv
[pytest]: https://github.com/pytest-dev/pytest