Standardize repo layout and CI

[zaneselvans]

We have a standard repository layout that we've adopted, which is reflected by the cheshire template repo. Apply it to the pudl-scrapers repository.

• [x] Create a src/ directory separating the installable packages from the top-level directory.
• [x] Move the tests/ directory to the top level of the repo.
• [x] Add an appropriate tox.ini configuration.
• [x] Add standard pre-commit hooks and address the issues they surface.
• [x] Swap pytest workflow for tox-pytest workflow.
• [x] Add bot-auto-merge workflow