Type checking a system-install package yields erronous results

Bug Report

I'm working on todoman a cli app which I have installed on my system since, well, it's a cli app that I use interactively daily.

I'm working on adding type hints to this project, so I have it installed the development code a virtualenv (pip install -e .) and also installed mypy and other developer tools in that virtualenv.

When I run mypy, it seems to read type definitions from my system-installed version of todoman, so type checking does not match what's inside the repository checkout.

In other words, when there's a from todoman import X, mypy seems to read the system library, and not the $(pwd)/virtualenv one. That one lacks type definitions, so the results are skewed.

To Reproduce

1. Install todoman using your system package manager (in my case pacman -S todoman ).
2. git clone --branch typing2 https://github.com/pimutils/todoman.git
3. cd todoman && mkvirtualenv todoman && pip install -e . && pip install mypy
4. Make sure nothing in caching your $PATH here and run which mypy.
5. mypy todoman.

Expected Behavior

mypy should fail here, since there's a type-checking error in the checked out brach.

Actual Behavior

mypy passes, since it seems to read the system install libraries which have no type hints.

Your Environment

➜ mypy --version
mypy 0.812

➜ cat .config/mypy/config
[mypy]
ignore_missing_imports = True
cache_dir = $HOME/.cache/mypy

➜ python --version
Python 3.9.5

➜ uname -srmo
Linux 5.12.6-arch1-1 x86_64 GNU/Linux

Reproduce the report with the listed pacman, editable-install, virtualenv, and mypy todoman steps, checking which mypy and the reported versions first. Trace why the system-installed package is selected instead of the checkout, and consider the issue done when the checked-out package is type-checked and the reported error is detected.