[docs] [issue43739] Fixing the example code in Doc/extending/extending.rst to declare and initialize the pmodule variable to be of the right type.

Change by Shreyan Avigyan <shreyan.avigyan@gmail.com>: ---------- nosy: +gvanrossum _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue43739> _______________________________________