squash! stdlib.run: Fix problems when an executable is missing

Make pylint happy (set noqa for E721 for the check the Fields is not
created directly).
The "isinstance" function has a different behaviour in this case
than we want.

leapp/models/fields/__init__.py

@@ -46,7 +46,7 @@ def __init__(self, default=None, help=None):  # noqa; pylint: disable=redefined-
         self._nullable = False
         self._default = default
 
-        if type(self) == Field:
+        if type(self) == Field:  # noqa;
             raise ModelMisuseError("Do not use this type directly.")
 
     def _validate_model_value(self, value, name):