Make PeachPy processes deterministic:

  https://github.com/Maratyszcza/PeachPy/issues/88
  https://issues.guix.gnu.org/50672

diff --git a/peachpy/name.py b/peachpy/name.py
index b6a03dc..c069fc2 100644
--- a/peachpy/name.py
+++ b/peachpy/name.py
@@ -95,6 +95,10 @@ class Namespace:
             self.prenames[scope_name.prename].add(scope)
 
     def assign_names(self):
+        # Step 0: sort the dictionary for deterministic output
+        self.prenames = dict(sorted(self.prenames.items(),
+                                    key=lambda item: "" if item[0] == None else item[0]))
+
         # Step 1: assign names to symbols with prenames with no conflicts
         for prename in six.iterkeys(self.prenames):
             if prename is not None: