Creating a news entry from the pull request as a separate commit.  Does this warrant an issue?

The installed set of packages is already fixed and no longer changing.
It is therefore safe to reuse a single `pkg_resources.WorkingSet` for
all invocations of `get_installed_version`.

Python 3 docs are more actively maintained and are the future of the
project.

Fixes https://github.com/pypa/pip/issues/5346

Offload more work to the underlying pip command used to install the
build requirements, so there's no need to duplicate code to handle
environment markers/extras. This is done by setting the correct options
from the finder passed in argument to `_install_build_reqs`.

If pip10 is installed and pyopenssl+cryptography are present in the
environment pip's vendored urllib3 will automatically activate its
pyopenssl adapter. This is by design so users can potentially opt-in to
more up-to-date TLS versions than what are provided by their copy of
Python. However, if a user has these packages & subsequently runs
pip install --ignore-installed cryptography then a segfault will occur
due to overwriting an mmap'd shared object.

The flag makes pip prefer an older but valid binary distributions over a newer source distributions.

Fixes #3785.

If there is an import from hamster, while we vendor ham, we should
not rewrite it.

Fixes https://github.com/pypa/pip/issues/5347

* Test that trailing os.sep are ignored while searching PATH
* Simplify paths list creation