Is it just for this particular svn.exe application, or is it for any application (e.g. tasklist.exe)? 

The symptoms sound like svn.exe is spawning a new instance of itself with the `CREATE_NEW_CONSOLE` creation flag, which allocates a new console and replaces the standard handles if they aren't explicitly set by STARTUPINFO. svn.exe could be doing this indirectly via ShellExecute[Ex] in order to elevate via the "runas" verb. Try running Python elevated (run as administrator), in which case svn.exe may behave normally.