If that is US_ASCII or None try the encoding from getdefaultlocale() if that was a "fallback" because python couldn't figure it out from the environment or OS, this wll result in None.
If not none, then make sure python actually knows about the encoding.
On the event of an error or unknown codec, revert to fallbacks
This is on darwin try utf-8 everything else ASCII. Also on the darwin and mac-roman, switch to utf-8 this was a bug on older pythons.
On a side note, it would seem that the encoding only matters for when a version of svn doesn't yet support --xml and for getting repository and svn versions numbers. The --xml should be utf8 per some messages on the SVN mailing lists. So if the version number are always 7-bit ASCII clean, it may be best to only support the file parsing methods for legacy SVNs, and support for svn without the command would simple go away as support for the older SVNs does.