flake8.api.legacy.get_style_guide invariantly triggers DeprecationWarning under python2

Minimal reproduction:

import warnings

from flake8.api.legacy import get_style_guide


warnings.simplefilter('error')
get_style_guide()

Stacktrace:

$ .tox/py27/bin/python test.py
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    get_style_guide()
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/api/legacy.py", line 31, in get_style_guide
    application.parse_configuration_and_cli([])
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/main/application.py", line 164, in parse_configuration_and_cli
    self.option_manager, argv
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/aggregator.py", line 47, in aggregate_options
    original_values.isolated)
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py", line 281, in parse
    return self.merge_user_and_local_config()
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py", line 244, in merge_user_and_local_config
    user_config = self.parse_user_config()
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py", line 216, in parse_user_config
    config = self.config_finder.user_config()
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py", line 120, in user_config
    config, found_files = self._read_config(self.user_config_file())
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py", line 56, in _read_config
    found_files = config.read(files)
  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/backports/configparser/__init__.py", line 696, in read
    stacklevel=2,
DeprecationWarning: You passed a bytestring as `filenames`. This will not work on Python 3. Use `cp.read_file()` or switch to using Unicode strings across the board.

The filename it is complaining about:

$ .tox/py27/bin/python -m pdb test.py
> /home/asottile/workspace/cheetah_lint/test.py(1)<module>()
-> import warnings
(Pdb) c
Traceback (most recent call last):
  File "/usr/lib/python2.7/pdb.py", line 1314, in main
    pdb._runscript(mainpyfile)
  File "/usr/lib/python2.7/pdb.py", line 1233, in _runscript
    self.run(statement)
  File "/usr/lib/python2.7/bdb.py", line 400, in run
    exec cmd in globals, locals

...

  File "/home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/backports/configparser/__init__.py", line 696, in read
    stacklevel=2,
DeprecationWarning: You passed a bytestring as `filenames`. This will not work on Python 3. Use `cp.read_file()` or switch to using Unicode strings across the board.
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/backports/configparser/__init__.py(696)read()
-> stacklevel=2,
(Pdb) up
> /home/asottile/workspace/cheetah_lint/.tox/py27/local/lib/python2.7/site-packages/flake8/options/config.py(56)_read_config()
-> found_files = config.read(files)
(Pdb) files
'/home/asottile/.config/flake8'

So looking at the DeprecationWarning, that's coming from configparser one of our dependencies. We can of course always ensure that our filenames are text before providing them to that library though.

Added bug:confirmed label

Milestone changed to %3.x.y

Added component:legacy-api label