Update pip to 18.1

[pyup-bot]

This PR updates pip from 18.0 to 18.1.

Changelog

18.1

=================

Features
--------

- Allow PEP 508 URL requirements to be used as dependencies.

As a security measure, pip will raise an exception when installing packages from
PyPI if those packages depend on packages not also hosted on PyPI.
In the future, PyPI will block uploading packages with such external URL dependencies directly. (`4187 <https://github.com/pypa/pip/issues/4187>`_)
- Upgrade pyparsing to 2.2.1. (`5013 <https://github.com/pypa/pip/issues/5013>`_)
- Allows dist options (--abi, --python-version, --platform, --implementation) when installing with --target (`5355 <https://github.com/pypa/pip/issues/5355>`_)
- Support passing ``svn+ssh`` URLs with a username to ``pip install -e``. (`5375 <https://github.com/pypa/pip/issues/5375>`_)
- pip now ensures that the RECORD file is sorted when installing from a wheel file. (`5525 <https://github.com/pypa/pip/issues/5525>`_)
- Add support for Python 3.7. (`5561 <https://github.com/pypa/pip/issues/5561>`_)
- Malformed configuration files now show helpful error messages, instead of tracebacks. (`5798 <https://github.com/pypa/pip/issues/5798>`_)

Bug Fixes
---------

- Checkout the correct branch when doing an editable Git install. (`2037 <https://github.com/pypa/pip/issues/2037>`_)
- Run self-version-check only on commands that may access the index, instead of
trying on every run and failing to do so due to missing options. (`5433 <https://github.com/pypa/pip/issues/5433>`_)
- Allow a Git ref to be installed over an existing installation. (`5624 <https://github.com/pypa/pip/issues/5624>`_)
- Show a better error message when a configuration option has an invalid value. (`5644 <https://github.com/pypa/pip/issues/5644>`_)
- Always revalidate cached simple API pages instead of blindly caching them for up to 10
minutes. (`5670 <https://github.com/pypa/pip/issues/5670>`_)
- Avoid caching self-version-check information when cache is disabled. (`5679 <https://github.com/pypa/pip/issues/5679>`_)
- Avoid traceback printing on autocomplete after flags in the CLI. (`5751 <https://github.com/pypa/pip/issues/5751>`_)
- Fix incorrect parsing of egg names if pip needs to guess the package name. (`5819 <https://github.com/pypa/pip/issues/5819>`_)

Vendored Libraries
------------------

- Upgrade certifi to 2018.8.24
- Upgrade packaging to 18.0
- Add pep517 version 0.2
- Upgrade pytoml to 0.1.19
- Upgrade pkg_resources to 40.4.3 (via setuptools)

Improved Documentation
----------------------

- Fix "Requirements Files" reference in User Guide (`user_guide_fix_requirements_file_ref <https://github.com/pypa/pip/issues/user_guide_fix_requirements_file_ref>`_)

Links

• PyPI: https://pypi.org/project/pip
• Changelog: https://pyup.io/changelogs/pip/
• Homepage: https://pip.pypa.io/

[pyup-bot]

Closing this in favor of #96