Don't add rowspan/colspan if it's 1.
Just a small thing I noticed in a [footnote here](https://danluu.com/web-bloat/#appendix-irony). Probably can't do much about the extra classes, but rowspan/colspan seem like easy fixes to save a few bytes per row/col and it's already done in the other code path.

Don't really know if this is major enough to deserve a what's new entry.

 - [ ] ~~closes #xxxx~~ - N/A
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry
