remove const declaration in @pydef which is deprecated in local scope (#571)

see JuliaLang/julia#23259

Author: marius311

src/pyclass.jl

@@ -150,7 +150,7 @@ Multiple dispatch works, too:
 """
 macro pydef(class_expr)
     class_name, _, _ = parse_pydef_toplevel(class_expr)
-    esc(:(const $class_name = $PyCall.@pydef_object($class_expr)))
+    esc(:($class_name = $PyCall.@pydef_object($class_expr)))
 end
 
 """