Prohibit access via class for instance-only attributes

[ilevkivskyi]

Fixes #240 (yes, you read that right, three digits)

I am taking a conservative approach and flag only cases that would definitely fail at runtime. Note we already have a more strict check for this for generics, but it is tricky to avoid double errors, so I think it is fine to have two errors sometimes.

I also update an outdated comment that confused me for a while.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

setuptools (https://github.com/pypa/setuptools)
+ setuptools/config/_apply_pyprojecttoml.py:328: error: Cannot access instance-only attribute "global_options" on class object  [misc]

[ilevkivskyi]

setuptools error is correct. There is just too much monkey-patching for mypy to figure out where the base class with the attribute comes from.