extensions: remove call to gettext.bind_textdomain_codeset().

I added this call originally, mimicking the C plug-ins. Yet in Python,
this function is deprecated since Python 3.8 and removed in 3.10.
Looking closer in the docs, it looks like Python's gettext module works
differently.

While all the *gettext() functions return strings in the locale's
encoding in C, unless overriden by bind_textdomain_codeset() (therefore
we need this call, in order to have locale-independent string encoding),
this is not the case in Python, whose gettext() call is already
locale-independent. The gettext.bind_textdomain_codeset() function only
applies to the specific l*gettext() functions which are deprecated too.

Fixes the warning:

> goat-exercise-py3.py:37: DeprecationWarning: bind_textdomain_codeset() is deprecated
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