gh-104783: locale.getlocale() calls sys.getfilesystemencoding()

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locale.getlocale() always calls sys.getfilesystemencoding(), instead of calling it only once.

• Issue: Remove locale.resetlocale() function in Python 3.13 #104783

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@methane: I added _encoding variable to be able to calle locale.getencoding() during Python finalization, when modules attributes are set to None. But maybe it's not needed. By the way, Python tries to keep sys attributes as long as posible. So I wrote this PR to change it :-)