Bug#969648: dask, pandas 1.1

[Rebecca N. Palmer]

Or maybe not an actual regression...it's a ~5e-7 difference and one of 
the things the patch does (at around 
dask/dataframe/tests/test_rolling.py:270) is _tighten_ the tolerance on 
that test.

I have filed a separate bug (#972516) for the fsspec issues.