PyOxidizer cannot find _posixsubprocess on Linux #406

With PyOxidizer v0.16.2 from PyPI for Python 3.9, running pip install with the PyOxidizer provided copy of Python during build raises:

ModuleNotFoundError: No module named '_posixsubprocess'
See the repro at https://github.com/layday/pyoxidizer-linux-subprocess (https://github.com/layday/pyoxidizer-linux-subprocess/runs/2665887808?check_suite_focus=true).