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/logger.py

@@ -18,7 +18,7 @@ import logging
 import os
 import platform
 import sys
-from cStringIO import StringIO
+from io import BytesIO
 
 # Some cool code for colored logging:
 # For background, add 40. For foreground, add 30
@@ -88,7 +88,7 @@ class WindowsOutputStream(object):
 
     def write(self, s):
 
-        msg_strm = StringIO(s)
+        msg_strm = BytesIO(s)
 
         while (True):
             c = msg_strm.read(1)