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.
With newer pillow versions, pillow would create debug log lines for
every single PNG images it touches, so that spams the log in
Overviewer's verbose mode quite a bit.
This also uses a dedicated logger instead of passing everything to the
root logger. `overviewer_core` still needs to be updated to reflect
this.
Additionally, `logger.configure` has been simplified.
