Enforce consistent code formatting in development

[ziw-liu]

Problem

Although we depend on the black formatter in the [dev] requirements, it is not currently consistently used, and there are unconventional formatting (wrt PEP 8) widespread in the code base.

Proposed solution

• We should arrange a PR that formats the existing code. (Done: Introduce Black formatting into the code base #229)
• The contributors should be required to format their code with the black formatter we specify in the [dev] requirement before pushing to the repository. (Development guide #185)
• (Optionally) we can also incorporate formatting formatting in our GItHub Actions workflow..

Example

black can be executed from the command line if the user has the [dev] environment activated.
If you are using a modern IDE, formatting can also be done by selecting the black installed in their virtual environment as the formatter backend (VS Code: ^/⌘ ⇧ P and type 'format document with...').

[ziw-liu]

Another issue to also think about is enforcing consistent Markdown documents with a linter, e.g. this one that is available as both a GitHub Action and an editor plugin.

[talonchandler]

Can this close @ziw-liu?

[ziw-liu]

Let's leave this for 1.0.0. CI checks for style and linting isn't implemented yet.