Fix imports for collections abstract classes

ebook_converter/constants_old.py

@@ -32,7 +32,7 @@ def debug():
 # plugins {{{
 
 
-class Plugins(collections.Mapping):
+class Plugins(collections.abc.Mapping):
 
     def __init__(self):
         self._plugins = {}

ebook_converter/css_selectors/ordered_set.py

@@ -19,7 +19,7 @@ def is_iterable(obj):
     return hasattr(obj, '__iter__') and not isinstance(obj, (str, bytes))
 
 
-class OrderedSet(collections.MutableSet):
+class OrderedSet(collections.abc.MutableSet):
     """
     An OrderedSet is a custom MutableSet that remembers its order, so that
     every entry has an index that can be looked up.