[lint] add the flynt linter for python f-strings

Summary:
Convert Python code from "%-formatted" and .format(...) strings into f-strings

Remove existing conversion script that only handles %-formatted strings.

Performance impact:

Before:

$ time arc lint --everything
real	2m15,395s
user	9m59,264s
sys	1m8,720s

After:

$ time arc lint --everything
real	2m14,881s
user	10m6,171s
sys	1m8,421s

Test Plan:
Add a few %-formatted "...".format() strings in a functional test, and check that arc lint fixes them.

arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13231