def rename(self, oldpath, newpath):
return SFTP_OP_UNSUPPORTED<EOL>
Rename (or move) a file.  The SFTP specification implies that this
method can be used to move an existing file into a different folder,
and since there's no other (easy) way to move files via SFTP, it's
probably a good idea to implement "move" in this method too, even for
files that cross disk partition boundaries, if at all possible.

@note: You should return an error if a file with the same name as
    C{newpath} already exists.  (The rename operation should be
    non-desctructive.)

@param oldpath: the requested path (relative or absolute) of the
    existing file.
@type oldpath: str
@param newpath: the requested new path of the file.
@type newpath: str
@return: an SFTP error code like L{SFTP_OK}.
@rtype: int