accept ∈ as a synonym for 'in' inside for loops and comprehensions

[stevengj]

As discussed in #8487, it's a bit odd and confusing that we allow x ∈ X as synonym for x in X comparisons, but we don't allow for x ∈ X as a synonym for for x in X.

This patch changes the parser to allow ∈ as a synonym for in in for loops and comprehensions.

(This should be backwards compatible: even existing code that works with ASTs is not affected, because both for x ∈ X and for x in X are parsed as for x = X.)

[mschauer]

Thanks. I saw that you change the example in the docs from "in" to "∈", I would keep the "in" for the moment as it is at this moment trivially more widely used as "∈".

[stevengj]

The docs have examples of both, as it should if both are supported. However, this PR is pending a conclusion to the discussion in #8487 of whether we want this syntax.

[JeffBezanson]

Sure, why not. Let's rebase and merge this.

[stevengj]

Rebased.