Backport PR #35852 on branch 1.1.x (API: replace dropna=False option with na_sentinel=None in factorize)
Backport PR #35852: API: replace dropna=False option with na_sentinel=None in factorize
@charlesdong1991 and can merge this when green as well > @charlesdong1991 and can merge this when green as well

and when merging on the backport branch ensure that Co-authored-by: Kaiqi Dong <kaiqi@kth.se> or whoever the author is is included otherwise they won't appear in the contributors list  may I kindly ask how to check if the author is included in this case? 😅  @simonjayhawkins  when you click on confirm squash and merge you'll see

![image](https://user-images.githubusercontent.com/13159005/92010074-42f17900-ed41-11ea-80c2-22f287c0055f.png)

so the the co-author is included automatically. just need to ensure that if cleaning the description that you don't remove the co-author (i.e. the original author) yep, get it! thanks very much! @simonjayhawkins 