Fix mypy in cpp_extension.py (#69101)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69101
Test Plan: Imported from OSS
Reviewed By: atalman, janeyx99
Differential Revision: D32730081
Pulled By: malfet
fbshipit-source-id: 76ace65b51850b74b175a3c4688c05e107873e8d
