pep8 changes to rigify (minor warnings still there)

also found quite a few cases where variables were used that hadnt been defined.

release/scripts/modules/rigify/stretch.py

@@ -19,8 +19,8 @@
 # <pep8 compliant>
 
 import bpy
-from rigify import get_layer_dict, RigifyError
-from rigify_utils import bone_class_instance, copy_bone_simple
+from rigify import RigifyError
+from rigify_utils import copy_bone_simple
 
 METARIG_NAMES = tuple()
 RIG_TYPE = "stretch"