vfs: add a NotImplementedError implementation of __call__

This helps pytype considerably on this file.

Differential Revision: https://phab.mercurial-scm.org/D7265

mercurial/vfs.py

@@ -54,6 +54,9 b' class abstractvfs(object):'
         '''Prevent instantiation; don't call this from subclasses.'''
         raise NotImplementedError('attempted instantiating ' + str(type(self)))
 
+    def __call__(self, path, mode=b'rb', **kwargs):
+        raise NotImplementedError
+
     def _auditpath(self, path, mode):
         raise NotImplementedError