Handle single element cases in PYI016

[tusharsadhwani]

Summary

For code like:

from typing import Union
x: Union[int, Union[int]]

PYI016 was not being raised, since the iterator was only matching on Tuples inside the Union[].

Same as PyCQA/flake8-pyi#484

Test Plan

cargo test

[tusharsadhwani]

Seems to have broken PYI055 pretty severely. The PYI041 failure seems like a false positive though!