Drop Python 3.5

[lesteve]

Python 3.5 has been dropped in distributed and dask 2.7.0 (dask/distributed@e4a0404 and dask/dask@0871699) i.e. November 2019.

We should probably do the same at one point.

[guillaumeeb]

Yep, no objection from me in the 0.8 release!