fix[tool]: fix classification of AST nodes (#4210)

fix classification of some AST nodes for proper type checking.

this is a hygienic fix, it should have no observable semantic changes.

vyper/ast/nodes.py

@@ -1053,7 +1053,7 @@ def _op(self, left, right):
             raise OverflowException(msg, self) from None
 
 
-class FloorDiv(VyperNode):
+class FloorDiv(Operator):
     __slots__ = ()
     _description = "integer division"
     _pretty = "//"
@@ -1334,7 +1334,7 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
 
-class AnnAssign(VyperNode):
+class AnnAssign(Stmt):
     __slots__ = ("target", "annotation", "value")