Forbid using `f` strings

I don’t see any reasons why we should use them. Let’s use .format() instead.

P.S. For a reason we can not forbid % usage: https://github.com/gforcada/flake8-pep3101/issues/23

[sobolevn]

.format() can be short too:

def some(self):
     return '{0} with {1}'.format(name, item)

But, for me being shorter is not a valid argument. What matters is:

1. readability
2. performance
3. consistency

Readability

My old habits put .format() over f, since I am just used to it. This is highly subjective, but that’s my point.

But, what really makes a difference is that we can not put logics inside .format(), while we can do bizarre things inside f strings:

>>> f'{random.randint(1, 12)}'
'3'

That is so wrong, that I can not allow this.

Performance

Yes, f is slightly faster than .format(). But it is safe to say that they are almost the same.

Consistency

There should be one-- and preferably only one --obvious way to do it. © Tim Peters

I don’t see any use-cases of f strings that can not be covered with .format(). So, let’s stick to a single way to format strings.

[kxepal]

I guess, @sobolevn really wants just linters for f-strings code and some heuristic to forbid doing complex things inside f-strings.