fix: remove .keyword from Call AST node (#3689)

for some reason, there is a slot named "keyword" on the Call AST node,
which is never used (and doesn't exist in the python AST!).

this commit removes it for hygienic purposes.

vyper/ast/nodes.py

@@ -1254,7 +1254,7 @@ def _op(self, left, right):
 
 
 class Call(ExprNode):
-    __slots__ = ("func", "args", "keywords", "keyword")
+    __slots__ = ("func", "args", "keywords")
 
 
 class keyword(VyperNode):