SQ -> WIP: Don't declare attributes on Expression to avoid confusing mypyc

Fix: Workaround mypy thinking incorrectly that isinstance(X, Union[...])
does not work at runtime.

Author: davidfstr

mypy/nodes.py

@@ -2151,7 +2151,7 @@ def accept(self, visitor: ExpressionVisitor[T]) -> T:
 # Expression subtypes that could represent the root of a valid type expression.
 # Always contains an "as_type" attribute.
 # TODO: Make this into a Protocol if mypyc is OK with that.
-MaybeTypeExpression = Union[IndexExpr, NameExpr, OpExpr, StrExpr]
+MaybeTypeExpression = (IndexExpr, NameExpr, OpExpr, StrExpr)
 
 
 class ComparisonExpr(Expression):