Don't populate a type arg for non generic managers

[flaeppe]

We probably want a more sophisticated implementation of this to get things right when a type has multiple arguments, specifically bounded, etc. But as a first step, this can at least eliminate the obviously invalid ones.

Related issues

• Refs Move ModelBase.objects declaration to Model.objects, for mypy 1.5.0 #1649 (comment)
• Refs Declare manager class attributes on models as ClassVars #1672 (comment)