Prevent more ways to assign to constants

[Kodiologist]

Fixes #1780.

[scauligi]

Test failures on 3.6, 3.7, pypy. My guess is because of this:

Changed in version 3.8: Class ast.Constant is now used for all constants.
Deprecated since version 3.8: Old classes ast.Num, ast.Str, ast.Bytes, ast.NameConstant and ast.Ellipsis are still available, but they will be removed in future Python releases.

So if we still plan to support these versions then you should include these classes in the new check in _storeize.

[Kodiologist]

Yep, thanks for the diagnosis. I'll look into it.

[Kodiologist]

It looks like the assignments are detected properly. The tests of the error messages are just too picky. So I changed the tests.