Files without extensions are able to be imported as Python files

Repro

In file.py:

import other

In other (no file extension):

#!/bin/sh

echo syntax error

Running:

$ mypy file.py
other:3: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

When running in CPython, you get this:

$ python file.py
Traceback (most recent call last):
  File "file.py", line 1, in <module>
    import other
ModuleNotFoundError: No module named 'other'

So I would expect Mypy to return either a "module not found" error or a "this is not a python file" error instead actually parsing other as a Python file. From reading the docs it would seem that the other module should only be importable from other.py[i] or other/__init__.py[i], but not a file called other.

I found this while running Mypy on beartype which has shell files called pytest and sphinx in the root directory, which causes Mypy to import the shell scripts if pytest or sphinx is not installed.

Environment

$ mypy --version
mypy 0.991 (compiled: yes)

$ python --version
Python 3.10.9

Reproduce the behavior with file.py importing the extensionless other file, using the mypy command shown in the issue, and compare it with CPython. Start with the documented import-discovery rules in the running_mypy import section; done means extensionless files are not parsed as Python modules and the resulting diagnostic matches the intended behavior.