[3.6] bpo-23894: make lib2to3 recognize f-strings (GH-1733) (#1737)

[3.6] bpo-23894: make lib2to3 recognize f-strings (GH-1733) (#1737)

Note: this doesn't unpack f-strings into the underlying JoinedStr AST.

Ideally we'd fully implement JoinedStr here but given its additional
complexity, I think this is worth bandaiding as is. This unblocks tools like
https://github.com/google/yapf to format 3.6 syntax using f-strings.
(cherry picked from commit 1b9530c536664276ce866ae602ce04adce0810e1)