vfs: add NotImplementedError version of join

Again, this helps out pytype.

Differential Revision: https://phab.mercurial-scm.org/D7266

mercurial/vfs.py

@@ -60,6 +60,9 b' class abstractvfs(object):'
     def _auditpath(self, path, mode):
         raise NotImplementedError
 
+    def join(self, path, *insidef):
+        raise NotImplementedError
+
     def tryread(self, path):
         '''gracefully return an empty string for missing files'''
         try: