feat(python): Rename Utf8 data type to String, keep Utf8 as alias

[stinodego]

I added the new String data type in three steps:

1. Rename the data type, add the alias, and update internal references / PyO3 bindings.
2. Run the test suite to make sure everything still works (with Utf8 used everywhere).
3. Update Utf8 to String in test suite too.

[ritchie46]

Does pl.Utf8() return pl.String()? I think that it should.

[stinodego]

Does pl.Utf8() return pl.String()? I think that it should.

No, it returns an object though that behaves exactly like String(). But I can make this change, there's something to be said for that. It's a very minor update - incoming :)

EDIT: Yeah if we accept that, things become much nicer 👍 we can just do Utf8 = String and export that. Just have to figure out how to nicely make Sphinx represent this.

[ritchie46]

Great! That makes it behave more like an alias.

[stinodego]

Yep, this is definitely better. Sphinx even picks this up automatically:

Should be good to go now!

[stinodego]

This is the core of this PR.

[ritchie46]

Have you done the datatypes mentioned in the user-guide?

Feel free to merge this one. Especially now it is a proper alias it seems fine to me.

[stinodego]

Have you done the datatypes mentioned in the user-guide?

Hadn't done that yet. Updated references in the user guide now. Will merge if CI is green!