pylint complains about unsubscriptable-object under Python ≥3.9

[gilles-peskine-arm]

In scripts/ci.requirements.txt, we impose a specific version of Pylint, because newer versions are likely to add additional checks that fail on our code. Unfortunately, newer versions of Python can also cause failures even with the same version of Pylint. Specifically, it looks like Pylint under Python ≥3.9 becomes somewhat type-aware, and complains about subscripting Optional[something_subscriptable].

python -m venv venv-3.9
. venv-3.9/bin/activate
scripts/min_requirements.py
tests/scripts/check-python-files.sh

results in many errors of the form

************* Module mbedtls_dev.build_tree
scripts/mbedtls_dev/build_tree.py:25:32: E1136: Value 'Optional' is unsubscriptable (unsubscriptable-object)
…

These are all very likely to be false positives since mypy, which has a smarter type checker, is happy.

The same version of Pylint under Python ≤3.8 is happy.

The solution for this specific problem is probably to disable unsubscriptable-object in .pylintrc. Let mypy do that job since it does it better.