No such file or directory for C module

[bryanforbes]

Starting with the following using mypy version 0.640:

$ python3 -m venv .venv
$ .venv/bin/pip install -U aiohttp mypy
$ echo "import aiohttp" > test.py

Running .venv/bin/mypy test.py leads to an error:

mypy: can't read file '/Users/bryan/Projects/mypy-repro/.venv/lib/python3.7/site-packages/aiohttp/_websocket': No such file or directory

This module is a C module that is imported like this:

if NO_EXTENSIONS:
    _websocket_mask = _websocket_mask_python
else:
    try:
        from ._websocket import _websocket_mask_cython  # type: ignore
        _websocket_mask = _websocket_mask_cython
    except ImportError:  # pragma: no cover
        _websocket_mask = _websocket_mask_python

[gvanrossum]

Yeah, I can repro this too. And it's a regression from 0.630. :-(