Python limited API: migrate PyImport_ExtendInittab

This patch replaces PyImport_ExtendInittab () with its limited C
API equivalent, PyImport_AppendInittab (), a convenience wrapper
around PyImport_ExtendInittab ().

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23830
Approved-By: Tom Tromey <tom@tromey.com>