fix(DataFrame): #799 to_dict

[cmp0xff]

• Closes DataFrame.to_dict(orient="index") should return a nested dictionary #799 (Replace xxxx with the Github issue number)
• Tests added: Please use assert_type() to assert the type of any return value

[Dr-Irv]

thanks @cmp0xff

[cmp0xff]

Hi @Dr-Irv, thank you for the approval. However, regarding #1283 (comment), do we have some guideline here?

[Dr-Irv]

Hi @Dr-Irv, thank you for the approval. However, regarding #1283 (comment), do we have some guideline here?

Not really. My attitude is that if we construct a set of tests that are allowing us to test proper usage, and reject improper usage, I'm fine with using Never this way. I should note that if a declaration returning Never is matched, then the type checkers aren't going to say "that's invalid", but they may or may not say that code that follows the improper usage won't execute.

Maybe one day the typing gods will agree on an approach to do "negative" typing.