Fixes #1955.

Two surgical config fixes for the mypy / flake8 linting setup under the Ubuntu noble linter image.

Root cause

The T499 errors are emitted by flake8-mypy, a flake8 plugin that wraps mypy with its own outdated args. The plugin has been deprecated since 2020 and is stuck on a Python 3.6 interpreter under noble. The plugin gets auto-loaded by flake8 because it's installed in the linter docker image.

.arclint already declares a separate, dedicated mypy linter that runs mypy --config-file=mypy.ini against the proper interpreter, so real mypy linting is happening regardless of flake8-mypy.

Changes

• .flake8rc: add T499 to the ignore list with an explanatory comment matching the style of the existing N802 / E999 notes. Silences the broken plugin without touching the linter image.
• • mypy.ini: bump python_version from 3.8 to 3.12 to match the project's actual Python target (WORKSPACE declares python_version = "3.12").
    This intentionally does not block on or imply the broader linter-replacement work tracked in Replace arcanist with a support linting tool #1896.

Tested

Walked through the diff against the issue body's repro. Have not run arc lint locally - Pixie's Bazel + linter docker setup isn't in my dev env. Happy for a maintainer to confirm the noise stops on a real run.