Support str param type for partition_cols in to_parquet function
- [x] closes #27117 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Not sure where to add whatsnew entry #PandasHack2019

@sbrice @DalynnHatch can you merge master; the code change is ok, but will comment on the tests after rebase. Closing as stale but @DalynnHatch ping if you'd like to pick back up and continue