Update constant_fold.py

Author: BobTheBuidler

mypyc/irbuild/constant_fold.py

@@ -10,7 +10,7 @@
 
 from __future__ import annotations
 
-from typing import Final, Union
+from typing import TYPE_CHECKING, Final, Union
 
 from mypy.constant_fold import constant_fold_binary_op, constant_fold_unary_op
 from mypy.nodes import (
@@ -26,15 +26,16 @@
     UnaryExpr,
     Var,
 )
-from mypyc.irbuild.builder import IRBuilder
 from mypyc.irbuild.util import bytes_from_str
+if TYPE_CHECKING:
+    from mypyc.irbuild.builder import IRBuilder
 
 # All possible result types of constant folding
 ConstantValue = Union[int, float, complex, str, bytes]
 CONST_TYPES: Final = (int, float, complex, str, bytes)
 
 
-def constant_fold_expr(builder: IRBuilder, expr: Expression) -> ConstantValue | None:
+def constant_fold_expr(builder: "IRBuilder", expr: Expression) -> ConstantValue | None:
     """Return the constant value of an expression for supported operations.
 
     Return None otherwise.