COMPAT: numpy test warnings
Fixes #30334.

Some of the warnings are coming from within pandas (like the FutureWarning on truediv), some are numpy warnings (like the nan-warning on sign). All in all changes are minor.

Tests are passing with `-Werror` for me now.
Hello @nschloe! 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 2020-01-20 19:27:10 UTC The truediv ones look perfect, thanks needs `black pandas/tests/plotting/test_datetimelike.py` @nschloe if you make a PR for just the truediv part id be comfortable merging that The truediv part alone won't help me much, but feel free to just take the code and include it. I'd rather keep working on this one here until it's in shape. can you merge master; we merged a patch which partially overlaps with this @jreback Done. @jbrockmendel if any comments can you rebase > can you rebase

There are conflicts now. Perhaps merge then? Merged master. Hi @nschloe  - sorry to chase you up, just wanted to ask whether you're still working on this :)
Seems that there's some test failures, but I can't check them as it says "build not found" Just pushed a tentative fix. cc @jbrockmendel  LGTM Thanks @nschloe 