Bug#969650: python3.9, dask, pandas 1.1

[Rebecca N. Palmer]

Control: severity 969648 serious
Control: tags 969650 pending
Control: tags 972033 pending

Python 3.9 related breakage has been declared RC, so if nobody objects, 
  I intend to upload pandas 1.1 to unstable (possibly tonight, but it 
probably won't build before numpy and matplotlib are binNMUd for Python 
3.9) despite the dask breakage.

As noted in #969648, it is likely that dask can be fixed, but this has 
not been tested in Debian.  (Testing dask also currently fails for lack 
of a Python 3.9 numpy.)