Pip: 「extra」がpip8.1.1で機能しない

• Pipバージョン：8.1.1
• Pythonバージョン：2.7.11+
• オペレーティングシステム：Ubuntu 16.04
  

  説明：

pip 8.1.1は、「UndefinedEnvironmentName： 'extra'は評価環境に存在しません」というエラーで失敗します。 extras_require（django-constance [database]）を使用するパッケージをインストールしようとしているとき

pip 8.1.0にダウングレードすると、問題が解決します。

私が実行したもの：

クリーンなvirtualenvで、setuptools == 21.0.0を使用して、実行しています

pip install 'django-constance[database]==1.1.2'

これは次のように失敗します：

Collecting django-constance[database]==1.1.2
  Downloading django_constance-1.1.2-py2.py3-none-any.whl
Exception:
Traceback (most recent call last):
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/basecommand.py", line 209, in main
    status = self.run(options, args)
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/commands/install.py", line 328, in run
    wb.build(autobuilding=True)
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/wheel.py", line 748, in build
    self.requirement_set.prepare_files(self.finder)
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/req/req_set.py", line 360, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/req/req_set.py", line 448, in _prepare_file
    req_to_install, finder)
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/req/req_set.py", line 387, in _check_skip_installed
    req_to_install.check_if_exists()
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/req/req_install.py", line 997, in check_if_exists
    self.satisfied_by = pkg_resources.get_distribution(self.req)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 535, in get_distribution
    dist = get_provider(dist)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 415, in get_provider
    return working_set.find(moduleOrReq) or require(str(moduleOrReq))[0]
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 943, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 808, in resolve
    if not req_extras.markers_pass(req):
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 993, in markers_pass
    return not req.marker or any(extra_evals) or req.marker.evaluate()
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/_vendor/packaging/markers.py", line 278, in evaluate
    return _evaluate_markers(self._markers, current_environment)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/_vendor/packaging/markers.py", line 203, in _evaluate_markers
    lhs_value = _get_env(environment, lhs.value)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/_vendor/packaging/markers.py", line 185, in _get_env
    "{0!r} does not exist in evaluation environment.".format(name)
UndefinedEnvironmentName: 'extra' does not exist in evaluation environment.

バージョン指定子なしで実行すると、別のエラーが発生します。

pip install 'django-constance[database]'

与える

Exception:
Traceback (most recent call last):
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/basecommand.py", line 209, in main
    status = self.run(options, args)
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/commands/install.py", line 305, in run
    wheel_cache
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/basecommand.py", line 270, in populate_requirement_set
    wheel_cache=wheel_cache
  File "/home/johnc/.virtualenvs/pip_8_1_1/local/lib/python2.7/site-packages/pip/req/req_install.py", line 234, in from_line
    extras).extras
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2833, in parse
    req, = parse_requirements(s)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2781, in parse_requirements
    yield Requirement(line)
  File "/home/johnc/.virtualenvs/pip_8_1_1/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2790, in __init__
    raise RequirementParseError(str(e))
RequirementParseError: Invalid requirement, parse error at "'__placeh'"

参照のためのジャンゴ-スタンスのsetup.py： https://github.com/jazzband/django-constance/blob/master/setup.py