Many things work, some don't. Notably, genPOI doesn't work, and
there's some signedness comparison stuff going on in the C extension.
This also completely drops support for Python 2, as maintaining a C
extension for both Python 2 and 3 is a pain and not worth it for the
9 months that Python 2 is still going to be supported upstream.
The documentation needs to be adjusted as well.
All of the few tests we have pass, and rendering a map works, both
with a configuration file and without. We can also use optimizeimages.
Concerns #1528.
pngnq-s9 is essentially just a more recently dead version of pngnq,
however, due to not being able to specify multiple binary names in
the past, we've not supported it. Refactor the code to allow specifying
multiple binary names, and add pngnq-s9 to it.
The validator will now warn if it detects that a crushed output
is fed into something that is not a crusher.
The is_crusher method of an optimizer shall return True if the
optimisation process is lossless, and does try to find optimal
encoding parameters as opposed to only removing unneeded channels
or reducing palettes.
