…n levels (#136594)

Fix the `test_generated_cases` to work with `-O` or `-OO` flags.
Previously, `test_generated_cases` was catching an `AssertionError` while `Tools/cases_generator/optimizer_generator.py` used an `assert` statement.  This approach semantically incorrect, no one should trying to catch an `AssertionError`!
Now the `assert` statement has been replaced with an explicit `raise ValueError(...)` and the corresponding `self.assertRaisesRegex(AssertionError, ...)` has been updated to catch a `ValueError` instead.

Co-authored-by: Nacho Caballero <nacho.caballero@astrazeneca.com>
Co-authored-by: Antonio Spadaro <ilovelinux@users.noreply.github.com>

Make the setlogmask() function in the syslog module thread-safe. These changes are relevant for scenarios where the GIL is disabled or when using subinterpreters.

…ules (GH-136846)

Pedantically reword the section about relative imports and main modules.