Reinforce in get_functions that eval/format methods are indeed callab…

…le (#1322)

`Builtin.get_functions` collects attributes of the subclasses with names starting with a prefix and it assumes that these are methods. 

Until now, the way to check that they are indeed methods was to check if they have a docstring. What we want to check is whether the method is callable. In Python 3.13, this is needed.
This PR implements a callable check.

mathics/core/builtin.py

@@ -468,6 +468,8 @@ def get_functions(self, prefix="eval", is_pymodule=False):
         for name in dir(self):
             if name.startswith(prefix):
                 function = getattr(self, name)
+                if not hasattr(function, "__call__"):
+                    continue
                 pattern = function.__doc__
                 if pattern is None:  # Fixes PyPy bug
                     continue