bpo-23894: make lib2to3 recognize f-strings (#1733)

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.
2 files changed