TYP: alias IndexLabel without Optional

[ivanovmg]

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Previously in _typing.py we had the alias IndexLabel, which contained Optional.
The present PR eliminates Optional from the alias, but adds it in the code where necessary.

Related to the issue raised by @simonjayhawkins in #36046 (comment)

[simonjayhawkins]

Thanks @ivanovmg lgtm pending green

[ivanovmg]

Travis CI failed at miniconda build. Can anyone restart?

[rhshadrach]

Since IndexLabel can be a Label, and Label = Optional[Hashable], isn't Optional[IndexLabel] the same as IndexLabel?

[jreback]

lgtm. @simonjayhawkins can you address @rhshadrach question.

[simonjayhawkins]

Since IndexLabel can be a Label, and Label = Optional[Hashable], isn't Optional[IndexLabel] the same as IndexLabel?

for typing, yes. just as Union[int, float] is equivalent to float and the Optional in function signatures could be implicit. We do this for clarity and consistency.

However, the actual convention that we should eventually adopt maybe determined by the outcome of #28585 and #33025.

If we go the route of showing the aliases in the docs ( scikit-learn/scikit-learn#16705 (comment) and numpy/numpydoc#196 related), having the Optional maybe preferable.

[jreback]

thanks @ivanovmg