DEPR: DataFrame.lookup
- [x] xref #18262
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This probably needs more discussion, but thought I open this PR as a starting point. In the xref you mention "Maybe a standalone function somewhere", is this still the idea or are we going to mention `melt` and `loc`? @jreback  no just put a nice example in the indexing docs - reference this in the deprecation
you can melt and use loc there

don’t need a standalone function Hello @erfannariman! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-09-16 17:58:12 UTC @WillAyd I put `Label-based "fancy indexing"` on top, but it still fails with the same message. The message is not really informative, "warning before the summary" really does not make sense to me. @erfannariman can you move the note to 1.2? Yes will do, but I am still strugling with the failing tests which I'm not sure how to solve. @WillAyd  thanks @erfannariman very nice!