Basically, the idea is to add the ability to mark abstract classes as strict.

What does this mean - in case when a class inherits from a strict abstract base class and doesn't implement all abstract methods then it will fail to create a class.

For instance:
>>> class Iterable(ABC, strict=True):
...   @abstractmethod
...   def __iter__(self):
...      pass

>>> class MyClass(Iterable):
...   pass
TypeError: Can't create class MyClass with unimplemented strict abstract method __iter__

That allows creating abc classes and be sure that class defines a required method, it will be perfrormed on a class declaration stage rather than class instance creation.