Remove Image.key, add Letter.key.

pixelocr/image.py

@@ -127,11 +127,6 @@ class Image(object):
     def isspace(self):
         return not _is_nonblank(self.bitmap)
 
-    @property
-    def key(self):
-        """Return a byte string uniquely representing the image."""
-        return self.tostring()
-
     def tostring(self):
         """Serialize the image as a string."""
         height, width, *_ = self.shape

pixelocr/page.py

@@ -165,6 +165,12 @@ class Letter(PageObject):
         super().__init__(image)
         self.elevation = elevation
 
+    @property
+    def key(self):
+        """Return a dictionary key uniquely representing this letter."""
+        return self.elevation, self.image.tostring()
+
+
 
 class Space(Letter):
     pass