bpo-29957: change LBYL key lookup to dict.setdefault (#938)

* change LBYL key lookup to dict.setdefault

The ``results`` was constructed as a defaultdict and we could simply
delete the check ``if key not in results``. However, I think it's safer
to use dict.setdefault as I'm not sure whether the caller expects a
regular dict or defaultdict.

* add name to the acknowledgements file

* use defaultdict to make the key-lookup cleaner

Lib/lib2to3/btm_matcher.py

@@ -117,10 +117,7 @@ class BottomMatcher(object):
                     #token matches
                     current_ac_node = current_ac_node.transition_table[node_token]
                     for fixer in current_ac_node.fixers:
-                        if not fixer in results:
-                            results[fixer] = []
                         results[fixer].append(current_ast_node)
-
                 else:
                     #matching failed, reset automaton
                     current_ac_node = self.root
@@ -134,8 +131,6 @@ class BottomMatcher(object):
                         #token matches
                         current_ac_node = current_ac_node.transition_table[node_token]
                         for fixer in current_ac_node.fixers:
-                            if not fixer in results.keys():
-                                results[fixer] = []
                             results[fixer].append(current_ast_node)
 
                 current_ast_node = current_ast_node.parent