flake8 >= 2.2.0 fails with pep8 1.5.7 due to pkg_resources.VersionConflict with mysterious empty list about flake8 HOT 5 CLOSED

$ mkvirtualenv --version
1.11.6
$ mktmpenv
$ pip install flake8 hacking
$ curl -O https://bootstrap.pypa.io/get-pip.py
$ flake8 get-pip.py
Traceback (most recent call last):
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/bin/flake8", line 9, in <module>
    load_entry_point('flake8==2.2.5', 'console_scripts', 'flake8')()
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/flake8/main.py", line 24, in main
    flake8_style = get_style_guide(parse_argv=True, config_file=DEFAULT_CONFIG)
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/flake8/engine.py", line 102, in get_style_guide
    kwargs['parser'], options_hooks = get_parser()
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/flake8/engine.py", line 55, in get_parser
    (extensions, parser_hooks, options_hooks) = _register_extensions()
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/flake8/engine.py", line 28, in _register_extensions
    checker = entry.load()
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/pkg_resources.py", line 2145, in load
    if require: self.require(env, installer)
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/pkg_resources.py", line 2159, in require
    items = working_set.resolve(reqs, env, installer)
  File "/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages/pkg_resources.py", line 643, in resolve
    raise VersionConflict(dist, req) # XXX put more info here
pkg_resources.VersionConflict: (pep8 1.5.7 (/Users/ian7708/virtualenv/tmp-362ac240fd778619/lib/python2.7/site-packages), Requirement.parse('pep8==1.5.6'))
$ python -c 'import setuptools; print(setuptools.__version__)'
3.6
$ pip install -U setuptools
$ flake8 get-pip.py
# same traceback
$ pip install -U --force-reinstall hacking
Downloading/unpacking hacking
  Downloading hacking-0.9.2-py2-none-any.whl
Downloading/unpacking pbr>=0.6,!=0.7,<1.0 (from hacking)
  Downloading pbr-0.10.0-py2.py3-none-any.whl (63kB): 63kB downloaded
Downloading/unpacking flake8==2.1.0 (from hacking)
  Downloading flake8-2.1.0.tar.gz
  Running setup.py (path:/Users/ian7708/virtualenv/tmp-362ac240fd778619/build/flake8/setup.py) egg_info for package flake8

Downloading/unpacking pyflakes==0.8.1 (from hacking)
  Downloading pyflakes-0.8.1-py2.py3-none-any.whl
Downloading/unpacking pep8==1.5.6 (from hacking)
  Downloading pep8-1.5.6-py2.py3-none-any.whl
Downloading/unpacking six>=1.6.0 (from hacking)
  Downloading six-1.8.0-py2.py3-none-any.whl
Downloading/unpacking pip (from pbr>=0.6,!=0.7,<1.0->hacking)
  Downloading pip-1.5.6-py2.py3-none-any.whl (1.0MB): 1.0MB downloaded
Downloading/unpacking mccabe>=0.2.1 (from flake8==2.1.0->hacking)
  Downloading mccabe-0.2.1.tar.gz
  Running setup.py (path:/Users/ian7708/virtualenv/tmp-362ac240fd778619/build/mccabe/setup.py) egg_info for package mccabe

Installing collected packages: hacking, pbr, flake8, pyflakes, pep8, six, pip, mccabe
  Found existing installation: hacking 0.9.2
    Uninstalling hacking:
      Successfully uninstalled hacking
  Found existing installation: pbr 0.10.0
    Uninstalling pbr:
      Successfully uninstalled pbr
  Found existing installation: flake8 2.2.5
    Uninstalling flake8:
      Successfully uninstalled flake8
  Running setup.py install for flake8

    Installing flake8 script to /Users/ian7708/virtualenv/tmp-362ac240fd778619/bin
  Found existing installation: pyflakes 0.8.1
    Uninstalling pyflakes:
      Successfully uninstalled pyflakes
  Found existing installation: pep8 1.5.7
    Uninstalling pep8:
      Successfully uninstalled pep8
  Found existing installation: six 1.8.0
    Uninstalling six:
      Successfully uninstalled six
  Found existing installation: pip 1.5.6
    Uninstalling pip:
      Successfully uninstalled pip
  Found existing installation: mccabe 0.2.1
    Uninstalling mccabe:
      Successfully uninstalled mccabe
  Running setup.py install for mccabe

Successfully installed hacking pbr flake8 pyflakes pep8 six pip mccabe
Cleaning up...
$ flake8 get-pip.py
get-pip.py:17420:13: F401 'setuptools' imported but unused

pip install flake8 hacking
Downloading/unpacking flake8
  Downloading flake8-2.2.5.tar.gz
  Running setup.py (path:/Users/ian7708/virtualenv/tmp-362ac240fd778619/build/flake8/setup.py) egg_info for package flake8

Downloading/unpacking hacking
  Downloading hacking-0.9.2-py2-none-any.whl
Downloading/unpacking pyflakes>=0.8.1 (from flake8)
  Downloading pyflakes-0.8.1-py2.py3-none-any.whl
Downloading/unpacking pep8>=1.5.7 (from flake8)
  Downloading pep8-1.5.7-py2.py3-none-any.whl
Downloading/unpacking mccabe>=0.2.1 (from flake8)
  Downloading mccabe-0.2.1.tar.gz
  Running setup.py (path:/Users/ian7708/virtualenv/tmp-362ac240fd778619/build/mccabe/setup.py) egg_info for package mccabe

Downloading/unpacking pbr>=0.6,!=0.7,<1.0 (from hacking)
  Downloading pbr-0.10.0-py2.py3-none-any.whl (63kB): 63kB downloaded
Downloading/unpacking six>=1.6.0 (from hacking)
  Downloading six-1.8.0-py2.py3-none-any.whl
Requirement already satisfied (use --upgrade to upgrade): pip in ./lib/python2.7/site-packages (from pbr>=0.6,!=0.7,<1.0->hacking)
Installing collected packages: flake8, hacking, pyflakes, pep8, mccabe, pbr, six
  Running setup.py install for flake8

    Installing flake8 script to /Users/ian7708/virtualenv/tmp-362ac240fd778619/bin
  Running setup.py install for mccabe

Successfully installed flake8 hacking pyflakes pep8 mccabe pbr six
Cleaning up...