Set npartitions in set_index

[jsignell]

• Tests added / passed
• Passes black dask / flake8 dask
• Fixes npartitions argument in set_index DataFrame method does not work. #6974

This causes some test failures, so I was hoping to get some advice on whether this is how npartitions is meant to be used on set_index

[rjzamora]

Hmm - The test_set_index_timezone failure may not be a problem. It seems that the test is calling d.set_index("notz", npartitions=1) and d.set_index("tz", npartitions=2), but getting a single partition for both cases anyway. There is no failure with npartition=1 (which is what the test was usually getting anyway).

[jsignell]

I did notice that it wasn't getting npartitions set properly. I'll try to fix the tests tomorrow.