MAINT: Fix warnings in radixsort.c.src: comparing integers of different signs
MAINT: Fix warnings in radixsort.c.src: comparing integers of different signs

Eliminate many compiler warnings of the form:

    numpy/core/src/npysort/radixsort.c.src:61:23: warning: comparison of integers of different signs: 'npy_intp' (aka 'long') and 'unsigned long' [-Wsign-compare]
            for (l = 0; l < sizeof(npy_ubyte); l++) {
                        ~ ^ ~~~~~~~~~~~~~~~~~
    numpy/core/src/npysort/radixsort.c.src:66:19: warning: comparison of integers of different signs: 'npy_intp' (aka 'long') and 'unsigned long' [-Wsign-compare]
        for (l = 0; l < sizeof(npy_ubyte); l++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~

numpy/numpy,grlee77/numpy,charris/numpy,charris/numpy,rgommers/numpy,pizzathief/numpy,charris/numpy,MSeifert04/numpy,pbrod/numpy,abalkin/numpy,mattip/numpy,pbrod/numpy,jorisvandenbossche/numpy,mhvk/numpy,abalkin/numpy,anntzer/numpy,mattip/numpy,WarrenWeckesser/numpy,MSeifert04/numpy,jorisvandenbossche/numpy,simongibbons/numpy,ahaldane/numpy,anntzer/numpy,charris/numpy,madphysicist/numpy,jakirkham/numpy,jakirkham/numpy,ahaldane/numpy,pbrod/numpy,pizzathief/numpy,ahaldane/numpy,simongibbons/numpy,madphysicist/numpy,pdebuyl/numpy,endolith/numpy,WarrenWeckesser/numpy,madphysicist/numpy,simongibbons/numpy,ahaldane/numpy,jakirkham/numpy,mhvk/numpy,endolith/numpy,numpy/numpy,pbrod/numpy,MSeifert04/numpy,rgommers/numpy,grlee77/numpy,mattip/numpy,jorisvandenbossche/numpy,anntzer/numpy,pizzathief/numpy,mattip/numpy,seberg/numpy,pdebuyl/numpy,jakirkham/numpy,mhvk/numpy,anntzer/numpy,grlee77/numpy,jakirkham/numpy,mhvk/numpy,endolith/numpy,grlee77/numpy,rgommers/numpy,numpy/numpy,jorisvandenbossche/numpy,mhvk/numpy,jorisvandenbossche/numpy,seberg/numpy,WarrenWeckesser/numpy,numpy/numpy,madphysicist/numpy,simongibbons/numpy,grlee77/numpy,seberg/numpy,ahaldane/numpy,MSeifert04/numpy,WarrenWeckesser/numpy,rgommers/numpy,pbrod/numpy,MSeifert04/numpy,endolith/numpy,seberg/numpy,pizzathief/numpy,pizzathief/numpy,pdebuyl/numpy,WarrenWeckesser/numpy,madphysicist/numpy,abalkin/numpy,pdebuyl/numpy,simongibbons/numpy