[Part B, Ch. 2] - Review and change example code

[Vasc01]

2.1. Single Responsibility Principle

• examples single_responsibility_bad.py and single_responsibility_good.py

2.2. Open/Closed Principle

• examples open_closed_bad.by and open_closed_good.by

2.5. Dependency Inversion Principle

• in file dependency_inversion_good.py make MathAbc an abstract class

[braboj]

Done, but for 2.5 we will not use abstract classes as they are a little bit more specific and require knowledge about the six library. The problem is due to the incompatibility when using metaclasses, required by the abc module.