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

Shreyan Avigyan <shreyan.avigyan@gmail.com> added the comment: I've managed to overcome the "HEAD Detached" problem I was facing when switching to the 3.9, 3.8, 3.7 and 3.6 branches. So I'm also submitting the PRs for those documentation versions. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue43739> _______________________________________