PredefinedSymbol -> SymbolConstant...

and try to clearify what is up here.

Author: rocky

mathics/builtin/makeboxes.py

@@ -5,8 +5,6 @@
 Low level Format definitions
 """
 
-from typing import Union
-
 import mpmath
 
 from mathics.builtin.base import Builtin, Predefined

mathics/core/symbols.py

@@ -333,7 +333,7 @@ class Symbol(Atom, NumericOperators, EvalMixin):
 
     All Symbols have a name that can be converted to string.
 
-    A Variable Symbol is a ``Symbol``` that is associated with a
+    A Variable Symbol is a ``Symbol`` that is associated with a
     ``Definition`` that has an ``OwnValue`` that determines its
     evaluation value.