mercurial/__init__.py

--- a/mercurial/__init__.py	Fri Mar 03 14:42:56 2017 -0500
+++ b/mercurial/__init__.py	Wed Mar 08 18:11:19 2017 -0500
@@ -137,6 +137,9 @@
             # Only handle Mercurial-related modules.
             if not fullname.startswith(('mercurial.', 'hgext.', 'hgext3rd.')):
                 return None
+            # zstd is already dual-version clean, don't try and mangle it
+            if fullname.startswith('mercurial.zstd'):
+                return None
 
             # This assumes Python 3 doesn't support loading C modules.
             if fullname in _dualmodules: