[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]

Change by Shreyan Avigyan <shreyan.avigyan at gmail.com>:


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nosy: +gvanrossum

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43739>
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