TYP: alias IndexLabel without Optional 
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] 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 https://github.com/pandas-dev/pandas/pull/36046#discussion_r485510442