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)

Start by reproducing the setup.cfg configuration with python_version = 3.5, the importlib_metadata dependency, test.py, and mypy test.py. Trace how mypy handles version-related syntax errors from imported dependencies; done means test.py still enforces Python 3.5 while the dependency's version-related errors are suppressed by ignore_errors.