[Python-Dev] transitioning from % to {} formatting

[INADA Naoki]

> -1 That requires keeping formatting information around in every string instance.
Adding new "format_string" class avoids it.
 unicode("foo") <=> u"foo"
 format_string("foo") <=> f"foo"

This way's pros:
* Many libraries can use one transition way.
* Transition stage syncs to Python version. "library A uses {} and
library B uses %" problem not happen in transition.
* We have experience same way on unicode.

-- 
Naoki INADA  <songofacandy at gmail.com>