Fix hardcoded directory name assumption

AFAICS this was broken by 710f22a
moving the source tree inside src/.  It wasn't very nice even before
that, to assume that the source will always be checked out inside a
directory named 'z3c.jbot'.

src/z3c/jbot/README.txt

@@ -61,7 +61,7 @@ Upon rendering, the global template manager will have reverted the
 template filename to the original.
 
   >>> view.template.filename
-  '.../z3c.jbot/z3c/jbot/tests/templates/example.pt'
+  '.../z3c/jbot/tests/templates/example.pt'
 
 Overrides can be registered for a specific layer. Let's register three
 more overrides, one for the general-purpose ``IRequest`` layer, one