Update stubgen.py

Author: BobTheBuidler

mypy/stubgen.py

@@ -57,6 +57,7 @@
 import mypy.util
 import mypy.version
 from mypy.build import build
+from mypy.constant_fold import constant_fold_expr
 from mypy.errors import CompileError, Errors
 from mypy.find_sources import InvalidSourceList, create_source_list
 from mypy.modulefinder import (