Pylint W0212 protected-access
pylint doesn't know of which type other is (how should it, you can compare an instance of A to everything), therefore the warning. I don't think there is a way around disabling the warning.
You can disable the warning for only that one line with appending # pylint: disable=W0212 to that line.
Christian Geier is right about why you're getting the error, and how to disable it.
I'd encourage you to consider changing your code, though: pylint is telling you something important. From your example code looks like you want to use eq compare objects of class A to other objects of class A, but your example won't guarantee that a caller won't try A() == C(). Returning True when you check Circle()._radius == Sphere._radius seems likely to cause problems.
See this stackoverflow thread for discussion of how to handle this.