wvtest.py: add a fallback definition of relpath() for Python < 2.6.

Since bup still supports Python 2.5 and relpath was added to Python in
2.6, add an os.path.relpath replacement, adapted from CPython source
(tag v2.6, file Lib/posixpath.py, hg-commit 95fff5a6a276).

Signed-off-by: Thomas Haller <thom311@gmail.com>
[rlb@defaultvalue.org: update LICENSE and add comment to code about
 updating LICENSE.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>