TST: DataFrame.to_parquet accepts pathlib.Path with partition_cols defined
- [x] closes #35902
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Does this require bumping the minimum pyarrow version? @jorisvandenbossche  hmm, how / why does this pass on older pyarrow? code lgtm just unclear on pyarrow versioning and why this would be working now - can you comment @arw2019 ? > code lgtm just unclear on pyarrow versioning and why this would be working now - can you comment @arw2019 ?

It was fixed on the pandas side in #35681. 

So it will work in 1.2 with pyarrow 0.15+ (I have checked current master against 0.15, 0.17 and 1.0 pyarrow) Thanks @arw2019 ! thanks @jorisvandenbossche @WillAyd @jreback for reviewing!