Upgrading pip 19 to 20

Removes the deprecated --unstable-feature flag.

Switches to the new dependency resolver by default.

Removes the --build-dir option, as per the deprecation.

Drops parallelization from pip list --outdated.

Removes the deprecated --skip-requirements-regex option.

Removes the interpreter-specific major version tag such as cp3-none-any from consideration.

Wheel processing no longer permits wheels containing more than one top-level .dist-info directory.

Disallows passing install-location-related arguments in --install-options.

Removes the undocumented and deprecated option --always-unzip.

Reverts building of local directories in place, restoring pre-20.1 behaviour of copying to a temporary directory.

Redacts authentication credentials from URLs in help messages.

Fixes passwords being visible in the index-url in the "Downloading <url>" message.

Adds support for Python 3.9.

Restores the --build-dir option as a no-op to soften the transition for tools that still use it.

pip freeze will stop filtering pip, setuptools, distribute and wheel from output in a future version; use --exclude to keep the old behavior.

Deprecates the --find-links option in pip freeze.

Deprecates support for Python 3.5.

Deprecates setup.py-based builds that do not generate an .egg-info directory.

Adds a deprecation warning for the invalid requirements format "base>=1.0[extra]".

Deprecates legacy setup.py install when building a wheel failed for source distributions without pyproject.toml.

Deprecates -b/--build/--build-dir/--build-directory, recommending TMPDIR/TEMP/TMP possibly with --no-clean.

Documents that pip 21.0 will drop support for Python 2.7.

Deprecates setup.py-based builds that do not generate an .egg-info directory.

Deprecates the undocumented --skip-requirements-regex option.

Deprecates passing install-location-related options via --install-option.

Deprecates the git+git@ form of VCS requirement, to be removed in pip 21.0; switch to git+https:// or git+ssh://.