Replace unneeded defaultdict with dict

sphinx/versioning.py

@@ -11,7 +11,6 @@
 """
 from uuid import uuid4
 from operator import itemgetter
-from collections import defaultdict
 
 from sphinx.util.pycompat import product, zip_longest
 
@@ -49,7 +48,7 @@ def merge_doctrees(old, new, condition):
     new_iter = new.traverse(condition)
     old_nodes = []
     new_nodes = []
-    ratios = defaultdict(list)
+    ratios = {}
     seen = set()
     # compare the nodes each doctree in order
     for old_node, new_node in zip_longest(old_iter, new_iter):