[Python] Type invariance in generic class when callable is involved

Why, in the following example, the type variable T in class E is invariant?

class C:
pass

class D(C):
pass

class E[T: C = C]:
def g(self, f: Callable[[], T]) -> T:
return f()

x: E = E[D]() # error


Callable return types are covariant. This typechecks:

x = E() # x is E[C]
x.g(D) # f is Callable[[], D] which fits on Callable[[], C]


Removing or otherwise changing the callable to not have T as a return type makes T covariant again:

class E[T: C = C]:
def g(self, f: Callable[[], None]) -> T:
return cast(T, f()) # anything

x: E = E[D]() # OK


Hence, considering the first example, what is the rationale for E[D] not be compatible with E[C]?

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