subprocess support

When running a `subprocess()` command in the context of a test case, the settings are not carried over into the subprocess, as it has its own execution context. While there's low likelihood that this library could or should prevent a non-Python command like `subprocess.check_call(["curl", "https://example.com"])`, if the subprocess is **another Python program**, we should attempt to apply the same settings to that Python execution. See how `pytest-cov` does this: https://pytest-cov.readthedocs.io/en/stable/subprocess-support.html