tools.calculation: replace deprecated ast.Num with ast.Constant

`ast.Num` and friends have been deprecated since Python 3.8, but only
started to generate warnings in the prereleases of Python 3.12.

Feels like stdlib made a bad trade here, simplifying Python internals
in exchange for users writing uglier, more complicated type checks such
as this one. But it's been so long, it's not as if they'll un-deprecate
the older, more-convenient-for-the-user classes.

sopel/tools/calculation.py

@@ -56,8 +56,11 @@ def _eval_node(self, node, timeout):
         A subclass could overwrite this to handle more nodes, calling it only
         for nodes it does not implement itself.
         """
-        if isinstance(node, ast.Num):
-            return node.n
+        if (
+            isinstance(node, ast.Constant) and
+            isinstance(node.value, (int, float))
+        ):
+            return node.value
 
         elif (isinstance(node, ast.BinOp) and
                 type(node.op) in self.binary_ops):
@@ -77,7 +80,7 @@ def _eval_node(self, node, timeout):
             return self.unary_ops[type(node.op)](operand)
 
         raise ExpressionEvaluator.Error(
-            "Ast.Node '%s' not implemented." % (type(node).__name__,))
+            "ast.Node '%s' not implemented." % (type(node).__name__,))
 
 
 def guarded_mul(left, right):