docs: def _validate(self, key, cls=None):
          """Verify the manifest schema."""
          if key not in self.manifest:
              raise ValueError("Manifest %s requires '%s'."
                               % (self.manifest_path, key))
          if cls:
              if not isinstance(self.manifest[key], cls):
                  raise TypeError("Manifest value '%s' should be %s, not %s"
                                  % (key, cls, type(self.manifest[key])))