Enable type checking and adding typing_copilot to dev/CI environment

This is to help track and discuss adding typing_copilot as a dev dependency, and as part of the CI for arviz.

TL;DR

We add type checking to pull request checks, and another tool to enforce that our type checking is only getting stronger.

Slightly longer

This is a discussion over whether we want to do two things:

1. Add a type check step to CI
2. Help migrate to this using typing_copilot

I’ve sketched some possible wins and losses as I see them, and I’m tagging @obi1kenobi (author of typing_copilot) who might be able to help us think through whether this makes sense. Note that he has already submitted #1397, #1398, #1491, #1492, #1493, and #1494. @OriolAbril and @obi1kenobi have some great conversations in there about what is going on, and what the benefits are.

The general idea of typing_copilot is that it checks your mypy configuration against the amount of checking you’ve already done, and will tighten the config file, if possible. To me, it seems like a no-brainer that if we decide type checking is a good idea, to use this.

Possible wins:

• Strong typing helps downstream users and maintainers
• Find existing bugs
• Extra layer of testing and compatibility (especially for arviz, which may have contributors for a specific library, unaware of conventions/return types elsewhere in the codebase)

Possible losses:

• Higher threshold for contributing / maintainers may need to get more familiar with typing to help contributors
• More noise during merges

[canyon289]

Do it 😄

[ColCarroll]

My initial thought, for what it is worth, is that we aim to get this in after the next release, and give it a test run. If nothing else, I bet the experience of a maturing code base adding typing will be an interesting experiment, and we’ll all learn something. It also seems easy to get rid of.