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/src/primitives/hide.c

@@ -52,9 +52,9 @@ hide_start(void *data, RenderState *state, PyObject *support) {
         for (i = 0; i < blocks_size; i++) {
             PyObject *block = PyList_GET_ITEM(opt, i);
             
-            if (PyInt_Check(block)) {
+            if (PyLong_Check(block)) {
                 /* format 1: just a block id */
-                self->rules[i].blockid = PyInt_AsLong(block);
+                self->rules[i].blockid = PyLong_AsLong(block);
                 self->rules[i].has_data = 0;
             } else if (PyArg_ParseTuple(block, "Hb", &(self->rules[i].blockid), &(self->rules[i].data))) {
                 /* format 2: (blockid, data) */