MNT: set stacklevel in the getfullargspec deprecation warning to 2 (GH-13029)

This is consistent with the rest of the `warnings.warn` usage in the
inspect.py module and aids identifying code that needs to be fixed.

This warning came in via d5d2b454

Lib/inspect.py

@@ -1112,7 +1112,7 @@ def getfullargspec(func):
     """
 
     warnings.warn("Use inspect.signature() instead of inspect.getfullargspec()",
-                  DeprecationWarning)
+                  DeprecationWarning, stacklevel=2)
     try:
         # Re: `skip_bound_arg=False`
         #