Style fixes

lib/spack/llnl/util/lang.py

@@ -486,7 +486,7 @@ class HashableMap(MutableMapping):
     """This is a hashable, comparable dictionary.  Hash is performed on
     a tuple of the values in the dictionary."""
 
-    __slots__ = 'dict',
+    __slots__ = ("dict",)
 
     def __init__(self):
         self.dict = {}