gh-106320: Remove private _PyLong converter functions (#108499)

Move these private functions to the internal C API (pycore_long.h):

* _PyLong_UnsignedInt_Converter()
* _PyLong_UnsignedLongLong_Converter()
* _PyLong_UnsignedLong_Converter()
* _PyLong_UnsignedShort_Converter()

Argument Clinic now emits #include "pycore_long.h" when these
functions are used.