Python 3.6 has reached its last beta where its features are frozen now.
It introduces a new syntactic element: f-string literals.
https://docs.python.org/3.6/whatsnew/3.6.html#whatsnew36-pep498

In f-string literals, the content inside braces are treated as real Python expressions with optional format string after a colon character like in format() protocol. The expression expansion may be done inside the format string as well though. (See the examples in the above link.)

Let's add support for this.