python3: don't symlink easy_install3 if it exists

python3 installs would fail while linking
prefix/share/python3/easy_install to easy_install3 if the target already
existed, which caused any python3 installation after the initial to
fail. Skip linking if the symlink already exists.

Library/Formula/python3.rb

@@ -102,7 +102,9 @@ def install
       system "#{bin}/python3.2", "setup.py", "install"
 
       # Symlink to easy_install3 to match python3 command.
-      ln_s "#{scripts_folder}/easy_install", "#{scripts_folder}/easy_install3"
+      if !(scripts_folder+'easy_install3').exist?
+        ln_s "#{scripts_folder}/easy_install", "#{scripts_folder}/easy_install3"
+      end
     end
   end