Use f-string instead of string.format

[Tadaboody]

New smell

Smelly code

"a={},b={}".format(a,b)

Fixed code

f"a={a},b={b}"

Potential gotchas:

• warn only on py3.6+
• if the expression has quotes in it make sure to alternate single quotes '' and double quotes ""
• Don't warn if the formatted string isn't called directly (if the string is waiting for later)

[Tadaboody]

Can look at https://github.com/asottile/pyupgrade which already implemented this for """insparation"""