[BE] use pathlib.Path instead of os.path.* in setup.py (pytorch#156742)

Resolves:

- pytorch#155998 (comment)

Pull Request resolved: pytorch#156742
Approved by: https://github.com/malfet

Author: XuehaiPan

.lintrunner.toml

@@ -122,6 +122,7 @@ is_formatter = true
 [[linter]]
 code = 'MYPY'
 include_patterns = [
+    'setup.py',
     'torch/**/*.py',
     'torch/**/*.pyi',
     'caffe2/**/*.py',