locale.getdefaultlocale() is deprecated

The `locale.getdefaultlocale()` function was deprecated in Python 3.11 and will be removed in 3.13 (see the note in [the library docs](https://docs.python.org/3/library/locale.html#locale.getdefaultlocale)). The deprecation warning issued when running says `use setlocale(), getencoding() and getlocale() instead`.

The only place it is used is crypto/encoding.py:

https://github.com/jupyterhub/nativeauthenticator/blob/3db8285e0b1f8296d41b1f5b1d2016852a50dc40/nativeauthenticator/crypto/encoding.py#L187-L198

I guess either `locale.getencoding()` or `locale.getpreferredencoding` would be a suitable replacement.

Read crypto/encoding.py at the usage linked around lines 187–198 and compare the deprecated locale.getdefaultlocale() call with the documented alternatives, including locale.getencoding() and locale.getpreferredencoding(). Confirm that the replacement removes the Python 3.11 deprecation warning and continues to provide the needed encoding behavior.

Written by the indexing model from the issue text.