Implement flake8-use-fstring

[Secrus]

https://github.com/MichaelKim0407/flake8-use-fstring

• FS001: % formatting is used.

• FS002: .format formatting is used.

• FS003: f-string missing prefix (ignored by default).

Config options:
--percent-greedy, --format-greedy

• Level 0 (default): only report error if the value before % or .format is a string literal.

• Level 1: report error if a string literal appears before % or .format anywhere in the statement.

• Level 2: report any usage of % or .format.

[charliermarsh]

I believe we support FS001 and FS002, at least partially, as UP031 and UP032, which also automatically upgrade % strings to .format strings, and .format strings to f-strings.

(I say "partially", because it doesn't flag every usage of % and .format strings -- it omits some cases depending on formatting.)

[charliermarsh]

We could change UP031 and UP032 to always flag those string formatting methods, even if they can't fix them. That'd be one way to close out this issue. But it's a bit of a behavior change for those rules.

[Secrus]

Let me test how it works right now, looking at the rules description, it might actually be enough.

[martinhoyer]

@charliermarsh Sorry for necro'ing a closed issue - just want to mention that in my recent usage, UP032 did indeed left some .format()s, which are a bit more complex then just already assigned variable(s).
I'm thinking an opt-in option in settings for it to flag them, even though auto-fix is not available could be nice?
Might include comment, like "consider adding helper variable for complex formatting before using it in f-string"