Avoid warning related to locale.getdefaultlocale deprecation

[pupitetris]

When tkfilebrowser is used inside a project, the code produces a DeprecationWarning due to the use of locale.getdefaultlocale.

See https://github.com/python/cpython/issues/90817

Solution based on getdefaultlocale's help and CPython's own code in the calendar.LocaleTextCalendar class. Commit:

https://github.com/python/cpython/commit/4fccf910738d1442852cb900747e6dccb8fe03ef