Fix Python 3 failure when constructing an egg_fetcher. Fixes #131.

CHANGES.txt

@@ -2,6 +2,12 @@
 CHANGES
 =======
 
+-----
+2.0.3
+-----
+
+* Issue #131: Fix RuntimeError when constructing an egg fetcher.
+
 -----
 2.0.2
 -----

setuptools/dist.py

@@ -295,7 +295,7 @@ class Distribution(_Distribution):
                 'find_links', 'site_dirs', 'index_url', 'optimize',
                 'site_dirs', 'allow_hosts'
             )
-            for key in opts.keys():
+            for key in list(opts):
                 if key not in keep:
                     del opts[key]   # don't use any other settings
             if self.dependency_links: