Improve docstring of known_hash in retrieve function

[BjoernLudwigPTB]

Specify that known_hash can also be None in the retrieve function
docstring. This change helps some IDEs to identify None as a valid value for
the known_hash argument.

My IDE complains about None as being not allowed as described in the corresponding issue.

Relevant issues/PRs:

Fixes #332.

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[BjoernLudwigPTB]

I am a bit surprised about the single pipeline failure. As I did not touch any code, I can hardly believe those failures are really related to my very minor change. Or is there something I missed?

[santisoler]

Thanks @BjoernLudwigPTB for this contribution! Don't worry, your change cannot trigger any test failure. Apparently the downloaded file got corrupted and the hash didn't matched, which proves that including checksums in Pooch was a great idea from the start. I rerun it. Let's wait until it passes and I'll merge this PR 🚀 .

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[BjoernLudwigPTB]

@santisoler Say, am I really invited, for such a small contribution, to join the AUTHORS.md? I would love to, but it feels a bit awkward as I imagine, most if not all the others did actually contribute much more useful things.

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