Cannot suppress python_version-related errors in third-party code

Bug Report

If i have python_version = 3.5 set, it's not possible to tell mypy to ignore related syntax errors it finds in dependencies.

Use case: i have code which needs to support python 3.5, as well as newer versions. I run mypy against it under various versions of python, with python_version = 3.5 set so that i don't accidentally introduce something which is incompatible (like variable annotations) which might not get caught until later. This has worked fine until some of the dependencies have started using variable annotations.

To Reproduce

1. Create a venv using python 3.6+.
2. pip install mypy importlib-metadata # Using importlib-metadata as an example lib which uses variable annotations
3. Create setup.cfg with:

[mypy]
python_version = 3.5

[mypy-importlib_metadata]
ignore_errors = True

1. Create test.py with:

import importlib_metadata

1. Run mypy test.py

Expected Behavior

mypy should enforce python_version = 3.5 for test.py, but it should ignore any version-related issues in importlib_metadata.

Actual Behavior

mypy fails with:

venv/lib/python3.8/site-packages/importlib_metadata/__init__.py:88: error: Variable annotation syntax is only supported in Python 3.6 and greater
Found 1 error in 1 file (errors prevented further checking)

mypy doesn't seem to treat this as a 'fatal' error, either, as setting pdb = True has no effect.

Your Environment

• mypy 0.790 and 0.800
• python 3.6.7, 3.6.12 and 3.8.5
• Debian Stretch and Linux Mint 20.1 (Equivalent to Ubuntu 20.04)

Inizia riproducendo la configurazione di setup.cfg con python_version = 3.5, la dipendenza importlib_metadata, test.py e mypy test.py. Traccia il modo in cui mypy gestisce gli errori di sintassi relativi alla versione provenienti dalle dipendenze importate; il lavoro è completato quando test.py continua a imporre Python 3.5 mentre gli errori relativi alla versione della dipendenza vengono soppressi da ignore_errors.