Don't allow unversioned migrationitems to be hashed

Unversioned migration items match different versioned items for the same
source, so hashing them will not produce a correct result.

Closes: #945471

britney2/hints.py

@@ -83,7 +83,8 @@ class Hint(object):
         if self.type != other.type:
             return False
         else:
-            return frozenset(self.packages) == frozenset(other.packages)
+            # we can't use sets, because unversioned items cannot be hashed
+            return sorted(self.packages) == sorted(other.packages)
 
     @property
     def type(self):

britney2/migrationitem.py

@@ -76,6 +76,10 @@ class MigrationItem(object):
         return isequal
 
     def __hash__(self):
+        if not self.version:
+            raise AssertionError("trying to hash unversioned MigrationItem: %s" %
+                                 (self.name))
+
         return hash((self.uvname, self.version))
 
     def __lt__(self, other):