For the moment, disable subvertpy.wc.WorkingCopy when used with Subversion 1.7.

For the moment, disable subvertpy.wc.WorkingCopy when used with Subversion 1.7.
Subversion 1.7 has some significant changes in the behaviour of
the working copy API because of the WC-NG rewrite.

Support for the WC API in newer versions of Subversion will be re-added later
(help welcome), but since most users (bzr-svn is the only I'm aware of)
don't seem to actually rely on it, it seemed better to disable it and
support 1.7 than to prevent use of newer svn versions altogether.