B020 false positives with iterable attribute acess

Both of these loops report that the iteration variable is being reassigned, which is it clearly not.

Both are a bit complex, but it is each to see that smoother has attribted accessed but not assigned to.

    for smoother in smoother.smoothers:
        train_basis = smoother.basis[train_index]
        train_der_basis = smoother.der_basis[train_index]
        train_der2_basis = smoother.der2_basis[train_index]
        train_cov_der2 = smoother.cov_der2
        # TODO: Double check this part. cov_der2 is calculated with all data
        train_x = smoother.x[train_index]

        train_smoothers.append(
            UnivariateGenericSmoother(
                train_x, train_basis, train_der_basis, train_der2_basis,
                train_cov_der2, smoother.variable_name + ' train'))

        test_basis = smoother.basis[test_index]
        test_der_basis = smoother.der_basis[test_index]
        test_cov_der2 = smoother.cov_der2
        # TODO: Double check this part. cov_der2 is calculated with all data
        test_x = smoother.x[test_index]

        test_smoothers.append(
            UnivariateGenericSmoother(
                test_x, test_basis, test_der_basis, train_der2_basis,
                test_cov_der2, smoother.variable_name + ' test'))

Here ax and series have attribute access but no assignment.

        for i, (ax, (series, def_name)) in enumerate(zip(axs, series)):
            if def_name != "residual":
                ax.plot(series)
            else:
                ax.plot(series, marker="o", linestyle="none")
                ax.plot(xlim, (0, 0), color="#000000", zorder=-3)
            name = getattr(series, "name", def_name)
            if def_name != "Observed":
                name = name.capitalize()
            title = ax.set_title if i == 0 and observed else ax.set_ylabel
            title(name)
            ax.set_xlim(xlim)

No source file or test is named in the issue. Reproduce the two loop examples and trace the B020 implementation to see how iterable attribute access is classified; done means the reported false positives no longer occur while genuine reassignment warnings remain covered by tests.