diff --git a/mercurial/__init__.py b/mercurial/__init__.py
--- a/mercurial/__init__.py
+++ b/mercurial/__init__.py
@@ -29,7 +29,7 @@
         """A sys.meta_path finder that uses a custom module loader."""
         def find_spec(self, fullname, path, target=None):
             # Only handle Mercurial-related modules.
-            if not fullname.startswith(('mercurial.', 'hgext.', 'hgext3rd.')):
+            if not fullname.startswith(('mercurial.', 'hgext.')):
                 return None
             # don't try to parse binary
             if fullname.startswith('mercurial.cext.'):
diff --git a/relnotes/next b/relnotes/next
--- a/relnotes/next
+++ b/relnotes/next
@@ -73,3 +73,7 @@
    `addunfinished()` in `state` module.
 
  * `cmdutil.checkunfinished()` now includes detection for merge too.
+
+ * We used to automatically attempt to make extensions compatible with
+   Python 3 (by translating their source code while loading it). We no
+   longer do that.