Include backward compatibility with older setuptools versions.

src/zc/buildout/easy_install.py

@@ -1668,7 +1668,15 @@ def _fix_file_links(links):
         yield link
 
 def _final_version(parsed_version):
+    try:
         return not parsed_version.is_prerelease
+    except AttributeError:
+        # Older setuptools
+        _final_parts = '*final-', '*final'
+        for part in parsed_version:
+            if (part[:1] == '*') and (part not in _final_parts):
+                return False
+        return True
 
 def chmod(path):
     if os.path.islink(path) or not os.path.exists(path):