extensions: attempt to use non-deprecated inspect method

Avoids some deprecation warnings when extension loading breaks.

Differential Revision: https://phab.mercurial-scm.org/D295

mercurial/extensions.py

@@ -186,7 +186,11 b' 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: