BUG: define SWIGPY_SLICEOBJECT

SWIG v 4.1.0+ use PyObject instead of PySliceObject after
swig/swig@e683168

This adds the definition of SWIGPY_SLICEOBJECT.

Author: nilason

Wrapping/Generators/Python/module_ext.i.in

@@ -6,8 +6,10 @@
 
 #if PY_VERSION_HEX >= 0x03020000
 # define SWIGPY_SLICE_ARG(obj) ((PyObject*) (obj))
+# define SWIGPY_SLICEOBJECT PyObject
 #else
 # define SWIGPY_SLICE_ARG(obj) ((PySliceObject*) (obj))
+# define SWIGPY_SLICEOBJECT PySliceObject
 #endif
 %}