def pytest_sessionstart() -> None:
    """Run once at the start of the test session."""
    warnings.filterwarnings("ignore", r".*no-sysmon")
    # Only in the main process...
    if WORKER == "none":
        # Create a .pth file for measuring subprocess coverage.
        pth_dir = find_writable_pth_directory()
        assert pth_dir
        sub_dir = pth_dir / "subcover.pth"
        sub_dir.write_text("import coverage; coverage.process_startup()\n", encoding="utf-8")
        # subcover.pth is deleted by pytest_sessionfinish below.
Run once at the start of the test session.
tests/conftest.py