python_runner: re-add os import

The os import is still required due to a recent Windows compatibility
change.

Change-Id: I7fbe520d41642a48739a4f0d5c1dc0b483bf650e
diff --git a/pw_build/py/python_runner.py b/pw_build/py/python_runner.py
index 043c35b..7780604 100755
--- a/pw_build/py/python_runner.py
+++ b/pw_build/py/python_runner.py
@@ -15,6 +15,7 @@
 
 import argparse
 import logging
+import os
 import pathlib
 import re
 import shlex