Add type hints and mypy config

[craigds]

Description

Adds type hints to the codebase with the goal of making mypy happy and enabling it as both a pre-commit hook and CI check.

checklist:

• [x] no mypy errors
• [x] add pre-commit hook
• [x] add CI check

Related links:

Checklist:

• [x] Have you reviewed your own change?
• [ ] Have you included test(s)?
• [ ] Have you updated the changelog?

[craigds]

getting there. 16 of the remaining errors are actually https://github.com/pallets/click/issues/2802