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.
Added resize_half() and resize_half_wrap() functions to composite.c and
overviewer.h, to replace the call to PIL's resize function made by
tileset.py. Also added "resize_half" to COverviewerMethods in main.c, so
it can be called from Python. Should increase performance by 10 to 20%
for the entire program.
