diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -186,7 +186,11 @@ def _runextsetup(name, ui):
             try:
                 extsetup(ui)
             except TypeError:
-                if inspect.getargspec(extsetup).args:
+                # Try to use getfullargspec (Python 3) first, and fall
+                # back to getargspec only if it doesn't exist so as to
+                # avoid warnings.
+                if getattr(inspect, 'getfullargspec',
+                           getattr(inspect, 'getargspec'))(extsetup).args:
                     raise
                 extsetup() # old extsetup with no ui argument
         except Exception as inst: