Add type hints to PyBIDS

[maximemulder]

Hello,

I am opening this issue to ask if there is any plan or motivation to add type hints (see PEP 484) to PyBIDS. I looked for issues with "type hints" or"typing" but I could not find anything on that matter.

Thanks.

[effigies]

PRs are welcome. I would suggest starting by getting a type checker running and passing in CI, and only after that start adding more than the bare minimum necessary annotations.

Please be sure to pin the specific version of the type checker, as they routinely break on new releases. It's better to have one failing PR to update the type checker than for all PRs to start failing.