#209 Fix build with SWIG 4.5

[Orion Poplawski]

SWIG 4.5.0 removed Python 2 compatibility macros (PyInt_, PyString_, SWIG_Python_str_*) from its runtime header pyhead.swg (see commit Python: Remove Python 2 compatibility macros). These macros mapped old Python 2 C API names to their Python 3 equivalents. Packages that reference them in custom SWIG typemaps or pre-generated wrapper code now fail to compile.

This patch replaces all occurrences with the corresponding Python 3 C API functions. The replacements are safe since the macros were already aliased to these exact functions in SWIG's Python 3 code path.

See https://src.fedoraproject.org/rpms/plplot/pull-request/17