fix up signedness in PyImport_ExtendInittab (#4831)

As a result of 92a3c6f493ad411e4cf0acdf305ef4876aa90669, the compiler complains:

Python/import.c:2311:21: warning: comparison of integers of different signs: 'long' and 'unsigned long' [-Wsign-compare]
    if ((i + n + 1) <= PY_SSIZE_T_MAX / sizeof(struct _inittab)) {
         ~~~~~~~~~  ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This overflow is extremely unlikely to happen, but let's avoid undefined
behavior anyway.