Upstream changes:
9.0.0 (2016-11-02)
BACKWARD INCOMPATIBLE Remove the attempted autodetection of requirement names from URLs, URLs must include a name via #egg=.
DEPRECATION pip install --egg have been deprecated and will be removed in the future. This "feature" has a long list of drawbacks where it breaks almost all of pip's other features in subtle and hard to diagnose ways.
Add a pip check command to check installed packages dependencies (PR #3750).
Added option to allow user to abort pip operation if file/directory exists
Added Appveyor CI
Uninstall existing packages when performing an editable installation of the same packages (#1548).
Pip show is less verbose by default. --verbose prints multiline fields. (PR #3858).
Added optional column formatting to pip list (#3651).
Add --not-required option to pip list to list packages that are not dependencies of other packages.
Fix the build on systems with symlinked /tmp directory for custom builds such as numpy (PR #3701).
Fix regression in pip freeze: when there is more than one git remote, priority is given to the remote named origin (PR #3708, #3616).
Fix crash when calling pip freeze with invalid requirement installed (PR #3704, #3681).
Support multiple --requirement files in pip freeze (PR #3703).
Implementation of pep-503 data-requires-python. When this field is present for a release link, pip will ignore the download when installing to a Python version that doesn't satisfy the requirement.
Pip wheel now works on editable packages too (it was only working on editable dependencies before); this allows running pip wheel on the result of pip freeze in presence of editable requirements (PR #3695, #3291).
Load credentials from .netrc files (PR #3715, #3569).
Add --platform, --python-version, --implementation and --abi parameters to pip download. These allow utilities and advanced users to gather distributions for interpreters other than the one pip is being run on. (PR #3760)
Skip scanning virtual environments even when venv/bin/python is a dangling symlink.
Added pip completion support for fish shell.
Fix problems on Windows on Python 2 when username or hostname contains non-ASCII characters (#3463, PR #3970, PR #4000).
Use git fetch --tags to fetch tags in addition to everything else that is normally fetched; this is necessary in case a git requirement url points to a tag or commit that is not on a branch (PR #3791)
Normalize package names before using in pip show (#3976)
Raise when Requires-Python do not match the running version and add --ignore-requires-python option as escape hatch (PR #3846).
Report the correct installed version when performing an upgrade in some corner cases (#2382)
Add -i shorthand for --index flag in pip search
Do not optionally load C dependencies in requests (#1840, #2930, #3024)
Strip authentication from SVN url prior to passing it into svn (PR #3697, #3209).
Also install in platlib with --target option (PR #3694, #3682).
Restore the ability to use inline comments in requirements files passed to pip freeze (#3680).
Deprecate --default-vcs option (#4052).
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