There are a few ways to deal with that. One is to declare the properties directly in the subclass:

  class Inject:
    def __init_subclass__(cls):
      cls.injected = 42

  class Child(Inject):
    injected: int

  print(Child().injected) # 42

This technique isn't too useful in practice, though it has its place (e.g. when creating ctypes.Structures). But there's a clever way to simplify this. Remember that the subclass is a subclass -- it inherits from Inject:

  import typing as t

  class Inject:
    injected: t.ClassVar[int]

    def __init_subclass__(cls):
      cls.injected = 42

  class Child(Inject):
    pass

  print(Child().injected) # 42

(ClassVar[int] ensures that type checkers know it's a variable attached to the class, not each instance.)