Future F-strings

Future F-strings is a library that backported f-string (formatted string literal) syntax to Python versions prior to 3.6. It enables developers to use f-strings in Python 2.7, and Python 3.3-3.5 environments by transforming the source code. The current version is 1.2.0, but the library is now deprecated as its target Python versions have reached end-of-life and MicroPython, a common target for its `rewrite` functionality, now supports f-strings natively.