Module has no attribute "__cached__"

Bug Report

Modules have a __cached__ attribute which points at the pyc location. mypy does not understand this attribute

Here's an example bit of code:

import copyreg
print(copyreg.__cached__)

OUTPUT = '''\
/usr/lib/python3.8/__pycache__/copyreg.cpython-38.pyc
'''

To Reproduce

(Write your steps here:)

1. Run mypy against the sample file

Expected Behavior

I expect no errors

Actual Behavior

$ mypy t.py
t.py:2: error: Module has no attribute "__cached__"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• sys.version: '3.8.5 (default, Jul 28 2020, 12:59:40) \n[GCC 9.3.0]'
• os.name: 'posix'
• sys.platform: 'linux'
• platform.system(): 'Linux'
• platform.python_implementation(): 'CPython'
• sys.implementation.name: 'cpython'
• sysconfig.get_platform(): 'linux-x86_64'

$ mypy --version
mypy 0.790

Start by creating or running the sample file t.py with mypy 0.790 and confirm the reported error for copyreg.cached. Trace how mypy represents module attributes, then add coverage for this case and verify that mypy reports no errors for the sample.