Make __all__ immutable

[tovrstra]

This is one on the list in #204, just a minor cleanup, to avoid making such changes along the way in future pull requests.

Making all a tuple is not a tradition in Python, but makes sense for things that are not supposed to be variable.

I will YOLO-merge this on Friday, June 5, unless reviewed earlier.

[deepsource-io]

Here's the code health analysis summary for commits 8cf1132..da392b4. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Shell	✅ Success		View Check ↗
Python	✅ Success		View Check ↗

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[PaulWAyers]

OK by me.

[tovrstra]

Thanks, merging...