[red-knot] Do not ignore typeshed stubs for 'venv' module (#16596)
## Summary
We currently fail to add the stubs for the `venv` stdlib module because
there is a `venv/` ignore pattern in the top-level `.gitignore` file.
## Test Plan
Ran the typeshed sync workflow manually once to see if the `venv/`
folder is now correctly added.
