Error when running on code inside venv containing `typing-extensions`

Bug Report

When running mypy on code installed in a virtualenv environment, mypy will fail outright if the environment contains typing-extensions.

To Reproduce

See repo containing a minimal example. The steps below describe the general idea:

1. Create a fresh virtual environment.
2. Create a minimal library with a setup.py that lists typing-extensions in install_requires.
3. Install the library to the virtual environment.
4. Run mypy on a file inside the environment that is part of the library (mypy venv/lib/python3.Y/site-packages/my_project/hello.py).

Expected Behavior

mypy would report errors in hello.py, if any, or exit with 0 if none.

Actual Behavior

mypy doesn't check hello.py, complaining that it found a typing_extensions.py in the environment:

mypy: "project-venv/lib/python3.6/site-packages/typing_extensions.py" shadows library module "typing_extensions"
note: A user-defined top-level module with name "typing_extensions" is not supported

Your Environment

• Mypy version used: 0.910.
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: multiple (3.6.8, 3.8.8).
• Operating system and version: CentOS 7

Beginne mit dem minimalen Beispiel im verlinkten mypy-typing-extensions-issue-Repository und reproduziere den Befehl mit einem in einer virtuellen Umgebung installierten Paket. Verfolge mypy's Module-Resolution-Einstiegspunkt für den gemeldeten Shadowing-Fehler und überprüfe anschließend, dass die Ziel-hello.py wie erwartet ohne den Fehler geprüft wird.