End support for python 3.5

[clbarnes]

EOL 2020-09-13 (see also PEP478) so no need to merge this in the next couple of days.

This will make pathlib use easier (see #601), among other things.

[joshmoore]

see also: zarr-developers/governance#11

[joshmoore]

Closing along with #603 (in order to open the Python 3.6 version)