refresh the tutorial about parent/element

[fchapoton]

by explicitly declaring sub-classes of Ring as no longer recommended.

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[tscrim]

One trivial change to emphasize more that there is not really any advantage to not using Parent.

On the other hand, there is a part of me that says we should just rewrite this altogether to use Parent (rather than putting it off).

What do you think?

[fchapoton]

well, the removal of all methods inside rings.pyx is still slowly progressing.. but yes, it could be better to switch the tutorial to use Parent. I have no strong opinion