@Serhiy I like the "convert to a tuple" idea before returning *better*, though I think I'd prefer to just check that it's tuple-like, something like:

    if not all(hasattr(return_val, attr) for attr in ['__getitem__', '__len__']) or len(return_val) != 2:
        warnings.warn("divmod must return a Sequence of length 2. " +
                      "This will be an exception in Python 3.9+",
                      Deprecationwarning)

Or some similar equivalent to `if not isinstance(return_val, collections.abc.Sequence) or len(return_val) != 2`.

That said, the PR seems to have already run into an issue, which is that `unittest.mock.MagicMock` returns a `unittest.mock.MagicMock` from `divmod`, which is not a 2-tuple.