pip 18.1 causes "TypeError: 'module' object is not callable"

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Issue description

New version of pip breaks pipenv, exactly on this line https://github.com/pypa/pipenv/blob/51cbc2e2f76a7c60f879a608fd96e95faca35f2e/pipenv/vendor/pip_shims/shims.py#L101

Expected result

The _strip_extras not be a module.

Actual result

...
 File "/opt/myproj/.tox/py36/lib/python3.6/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 704, in from_line

    line, extras = _strip_extras(line)

TypeError: 'module' object is not callable

Steps to replicate

Provide the steps to replicate (which usually at least includes the commands and the Pipfile).

---

Please run $ pipenv --support, and paste the results here. Don’t put backticks (`) around it! The output already contains Markdown formatting.

If you’re on macOS, run the following:

$ pipenv --support | pbcopy

If you’re on Windows, run the following:

> pipenv --support | clip

If you’re on Linux, run the following:

$ pipenv --support | xclip

[nealedj]

Workaround for typical virtualenv setup:

pip install pipenv
pipenv run pip install pip==18.0
pipenv install

[uranusjr]

@Froskekongen Please try to avoid non-constructive messages. We understand this is frustrating, but interrogation doesn’t help. If you feel we need to improve, consider offering help instead of shouting at the sideline. “We” (whoever you are referring to) are always short of resources, and would gladly use your help.