[zstd] Improve typing for zstd_dict parameter

[Rogdham]

Adding tuple[ZstdDict, int] to the possible values of the zstd_dict parameter.

Per the documentation:

When passing a ZstdDict to a function, the as_digested_dict and as_undigested_dict attributes can control how the dictionary is loaded by passing them as the zstd_dict argument, for example, compress(data, zstd_dict=zd.as_digested_dict).

…and both as_digested_dict and as_undigested_dict attributes of ZstdDict give a tuple[ZstdDict, int].

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Rogdham]

I noticed that the CPython documentation mentions for all these parameters that they must be a ZstdDict instance or None. But latter the part about Zstandard dictionaries says that we actually can pass zd.as_digested_dict or zd.as_undigested_dict.

I guess this is on purpose, to make the documentation easier to read.

@emmatyping 👋

[emmatyping]

I guess this is on purpose, to make the documentation easier to read.

Yep! I think it would be harder to understand the high level APIs if we mentioned the (Un)digested dictionaries but if you have wording you think will work I'd be happy to review it!

[Rogdham]

Thank you for the reviews & merge! 🤗