Initial Python 3 port

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.

overviewer_core/optimizeimages.py

@@ -34,8 +34,7 @@ class Optimizer:
         path = os.environ.get("PATH").split(os.pathsep)
 
         def exists_in_path(prog):
-            result = filter(lambda x: os.path.exists(os.path.join(x, prog)),
-                            path)
+            result = [x for x in path if os.path.exists(os.path.join(x, prog))]
             return len(result) != 0
 
         binaries = self.binarynames + [x + ".exe" for x in self.binarynames]