PEP 675: Add explicit `circumference` method (#2618)

Not adding a `radius` attribute, because the Circle class needs to be
instantiable from just the `scale` config value. So, I'm going with a
method.

pep-0673.rst

@@ -227,7 +227,8 @@ value of type ``Shape``, when in fact it should be ``Circle``:
 
 ::
 
-    class Circle(Shape): ...
+    class Circle(Shape):
+        def circumference(self) -> float: ...
 
     shape = Shape.from_config({"scale": 7.0})
     # => Shape