GH1037 Remove na_sentinel from factorize methods

[loicdiridollou]

• Closes Removed na_sentinel argument from factorize(), Index.factorize(), and ExtensionArray.factorize() #1037
• Tests added: Please use assert_type() to assert the type of any return value

[Dr-Irv]

To close the issue, you will have to also edit pandas-stubs/core/base.pyi as well. Basically anywhere that def factorize() appears, and then reconcile it with the docs.

[loicdiridollou]

All should align with the docs now, let me know if anything extra is needed.

[Dr-Irv]

can you also update core/base.pyi that has a the declaration for Index.factorize() ?

[loicdiridollou]

Aahh I see what you mean, sorry I got confused, added a test on the side, the type hinting seems a bit broad on it but that is for another day.
Now it aligns with the docs https://pandas.pydata.org/docs/reference/api/pandas.Index.factorize.html#pandas.Index.factorize