Fix macOS compile with newer clang

[dopplershift]

This fixes up a compiler error seen on conda-forge with >=v15 of the Mac compilers, stemming from:

 src/pygrib/_pygrib.c:44829:3: error: incompatible pointer to integer conversion returning 'void *' from a function with result type 'int' [-Wint-conversion]
    import_array();
    ^~~~~~~~~~~~~~
  /Users/runner/miniforge3/conda-bld/pygrib_1698064180141/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_pl/lib/python3.10/site-packages/numpy/core/include/numpy/__multiarray_api.h:1545:151: note: expanded from macro 'import_array'
  #define import_array() {if (_import_array() < 0) {PyErr_Print(); PyErr_SetString(PyExc_ImportError, "numpy.core.multiarray failed to import"); return NULL; } }
                                                                                                                                                        ^~~~
  /Users/runner/miniforge3/conda-bld/pygrib_1698064180141/_build_env/lib/clang/16/include/stddef.h:89:16: note: expanded from macro 'NULL'
  #  define NULL ((void*)0)
                 ^~~~~~~~~~

I see this building git locally, but only as a warning. Regardless, it seems easier to just call the Python function in the API instead of the macro.

I also fixed up some const warnings, including updating a few function devs from upstream.

[dopplershift]

Ok, this time the build worked properly. (ping @jswhit)