flake8 e999 on using fstrings in python2 (with future_fstrings)

I'm using fstrings in python2 after pip install future-fstrings like the following:

# -*- coding: future_fstrings -*-
class A(object):
    def __init__(self, **kw):
        self.__dict__.update(**kw)
        print (f'{self.a}')
        print (f'{self.b}')
        print (f'{self.c}')

A(a=1,b=2,c=3)

However, when commiting such a file to my project I get:

flake8 (Python 3.6 syntax check).....................(no files to check)Skipped
flake8 (Python 2.7)......................................................Failed
hookid: flake8

file_name:0:1: E999 SyntaxError: unknown encoding: future_fstrings

Why is this happening and how can I fix it?

# noqa E999 doesn't help...