Let's see the following code:
>>> int.__mro__
(<class 'int'>, <class 'object'>)
>>> import numbers
>>> issubclass(int, numbers.Number)
True
We can see there is no Number in __mro__, that means int does not inherit from Number, but why int is subclass of the Number?
Number is an abstract base class (ABC). Normal types can be registered as subclasses of the ABC.
See https://docs.python.org/3/library/abc.html#abc.ABCMeta.register :
register(subclass)
Register subclass as a “virtual subclass” of this ABC.
The numbers module contains the line
Integral.register(int)
Integral is a subclass of Rational < Real < Complex < Number.
Therefore int is a virtual subclass of Number.