Drop Python 2, 3.4, 3.5

[azmeuk]

Should fix #552

[davidism]

We can also remove 3.5, using pyupgrade --py36-plus hits most of it, but f-string conversion doesn't catch all the cases so I usually do some manual search and cleanup.

[azmeuk]

Python 3.5 is still supported until 2020-09-13.
Don't you think it worth waiting a few months before dropping py35 support?

[davidism]

We decided with Pallets to just drop it early.

[davidism]

Since this is in master, 2.3 will still support the old versions, so people will have a relatively updated version to use if they can't upgrade. I'm fine with dropping 3.5 in master.

[azmeuk]

Fair enough :)

[azmeuk]

Done.