Fixes issue #1556

Replaced XXX.keys() calls with idiomatic python 3 calls that are compatible with python 2 such as eg:
 * replaced "y = xxx.keys()" or "y = list(xxx.keys())" with just "y = list(xxx)"
 * replaced "sorted(xxx.keys())" or "sorted(list(xxx.keys()))" with just "sorted(xxx)"
 * replaced "if not A in B.keys():" with "if A not in B:"
 * replaced "for A in B.keys():"  with "for A in B:"

See also https://python-future.org/compatible_idioms.html
https://python-future.org/compatible_idioms.html#dict-keys-values-items-as-a-list

wx/lib/inspection.py

@@ -762,7 +762,7 @@ class FlagsFormatter(object):
 
     def __str__(self):
         st = []
-        for k in self.d.keys():
+        for k in self.d:
             if self.val & k:
                 st.append(self.d[k])
         if st: