Fix build for Python 3.13

[justvanrossum]

I'm not sure what (dependency?) caused this, but I currently can't build openstep_plist on macOS with Python 3.13.5:

      Error compiling Cython file:
      ------------------------------------------------------------
      ...
              elif isinstance(obj, bool):
                  self.dest.push_back(c'1' if obj else c'0')
                  return 1
              elif isinstance(obj, float):
                  return self.write_short_float_repr(obj)
              elif isinstance(obj, (int, long)):
                                         ^
      ------------------------------------------------------------
      
      src/openstep_plist/writer.pyx:200:35: undeclared name not builtin: long

I noticed while I tried to build for Free-threaded Python, which which we don't build wheels yet.

I'm assuming the 'long' thing is a remnant from Python 2.