Unary plus operator (Issue #16073)
 - [x] closes #16073
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Continuation from https://github.com/pandas-dev/pandas/pull/16103. 
Hopefully I've addressed all your concerns and rebased. can you rebase / update can you move whatsnew & rebase / update closing as stale. if you want to re-visit, pls comment and we can re-open.