ENH: Add minimal type annotations and run mypy in CI

[effigies]

This PR aims to get us started down the path of type annotations (#1109) by getting to a minimal working mypy check.

These annotations are not intended to be comprehensive, but resolved mypy errors in what seemed the most correct and succinct way.

Thanks to @simkarwin's #1152 I've learned that we can use from __future__ import annotations to use Python 3.10-style annotations in a code-base that supports Python 3.7, which is quite nice.

I've shielded the tests/ directories from type checking for now. Resolving issues there will probably help stress test our annotations, but it's a bit much for my current attention span.

[codecov]

Codecov Report

Base: 92.38% // Head: 92.38% // No change to project coverage 👍

Coverage data is based on head (02c7a34) compared to base (cfa209a).
Patch has no changes to coverable lines.

❗ Current head 02c7a34 differs from pull request most recent head 85f4cb4. Consider uploading reports for the commit 85f4cb4 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
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  Coverage   92.38%   92.38%           
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  Files          97       97           
  Lines       12250    12250           
  Branches     2525     2525           
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  Hits        11317    11317           
  Misses        613      613           
  Partials      320      320           

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[effigies]

If anybody's feeling typish, I would appreciate a review here.

[effigies]

I plan to merge this in the morning. If someone wants to review first but hasn't found time, let me know.

[fepegar]

This is great. Thanks, @effigies!

[fepegar]

I'm still getting Skipping analyzing "nibabel": module is installed, but missing library stubs or py.typed marker for the latest release. Do you need a py.typed file?

• https://dev.to/whtsky/don-t-forget-py-typed-for-your-typed-python-package-2aa3

[fepegar]

Happy to contribute, if you do.

[effigies]

Please do!

[effigies]

Fernando, I went ahead and opened #1196.