[config] protect is_serializable against circular references

[picnixz]

I'm not entirely sure of the coverage since it's very hard to check things or generate examples of "broken" configurations that would be meaningful.

Note that it is not possible to test equality of objects with circular references using == (thus, we cannot test equality of objects with circular references after pickle+unpickle since the references are not the same).

It does not fix #11752 because the recursion error occurs in the __instancecheck__ and I was not given anything to reproduce (so I cannot say for sure that it was directly related to that or if the recursion error happened at that call site). However, we do have an issue with circular objects (if any).

[picnixz]

Forgot to add a CHANGES entry but I'll do it next week (I'm leaving for today).

[picnixz]

I may also add a simple test for is_serializable without circular references, just to check that I did not mess up a possible condition.

[chrisjsewell]

thanks @picnixz, at a quick high-level look I think this looks all good

[picnixz]

I'm not entirely confident that the recursive guard based on the id is really sufficient but I cannot come up with a counter-example. Intuitively, we are not modifying the objects being tested during the check so even if they are mutable containers, since we are only checking their type, it should not be an issue. However, if the collections happens to be mutated (without changing its length) for whatever reason, then there might be an issue.

Now, I will not make it entirely foolproof because, as the name indicates, I don't want to protect against fools too much.