Add pre-commit configuration to apply Black formatting

[andrewfrench]

Adds local pre-commit configuration to enforce Black formatting. If code is not formatted appropriately, the Code Checks workflow will fail. The README has been updated with info on formatting expectations and how to configure pre-commit, which has been added as a dependency using poetry.

Configure pre-commit using poetry:

poetry run pre-commit install

⚠️ Note: this PR also increases the line length configuration from 80 to 120. The large diff is due to running Black after making this change.

Closes #426

[collindutter]

I had originally named this code-checks.yml because I thought we might have other checks (linting, static type checking) in the future. But maybe it's a good idea to separate workflows that make direct changes to the branch vs workflows that just check it.

[andrewfrench]

Black formatting was the only job in the workflow, so I felt the name was misleading once it was updated to autocommit. If we want to keep code formatting/style/static analysis stuff in this file I can change the name back.

[andrewfrench]

Retained the original Code Checks naming as this now uses pre-commit and PR checks rather than autocommits.

[collindutter]

@andrewfrench based on the number of files changed I assume you also changed the line length to 120? If so, let's update the description to reflect that.