REF: pass str_rep through arithmetic ops more consistently
This doesn't get all of the places where we fail to pass str_rep, still working out a couple of places where passing it breaks tests.
Hello @jbrockmendel! 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 2020-02-22 15:50:32 UTC closing to clear the queue re-opening, this is a step on the path to doing frame-with-frame ops blockwise lgtm. rebase ping on green. rebased+green thanks