I added the expr_text optional field to the FormattedValue node in order to implement the '=' feature of f-strings (see issue 36817).

However, the expr_text field isn't strictly needed. Instead, the same feature could be added with another Constant string node child of the JoinedStr node.

I'm going to remove expr_text and use another Constant in order to remove this change to the 3.8 ast nodes. I have a patch mostly worked out, I'll have it ready in the next day or two. I want to get this in to 3.8 beta 1, because otherwise we're stuck with the expr_text implementation.