__NewStyleClassMixIn => _NewStyleClassMixIn

One underscore is sufficient to indicate that the class is "private". Unlike a method or attribute, double-underscore in a class name doesn't result in name mangling.
```
************* Module salt.log
C0103:141,0:__NewStyleClassMixIn: Invalid name "__NewStyleClassMixIn" for type class (should match [A-Z_][a-zA-Z0-9]+$)
```