Pex

This release introduces the ability to copy requirements from an
existing PEX into a new one.

This can greatly speed up repeatedly creating a PEX when no requirements
have changed. A build tool (such as Pants) can create a "requirements
PEX" that contains just a static set of requirements, and build a final
PEX on top of that, without having to re-run pip to resolve
requirements.

* Support for copying requirements from an existing pex. (948)

This release brings enhanced performance when using the Pex CLI or API
to resolve requirements and improved performance for many PEXed
applications when specifying the `--unzip` option. PEXes built with
`--unzip` will first unzip themselves into the Pex cache if not unzipped
there already and then re-execute themselves from there. This can
improve startup latency. Pex itself now uses this mode in our [PEX
release](
https://github.com/pex-tool/pex/releases/download/v2.1.8/pex).

* Better support unzip mode PEXes. (941)
* Support an unzip toggle for PEXes. (939)
* Ensure the interpreter path is a file (938)
* Cache pip.pex. (937)

This release brings more robust control of the Pex cache (PEX_ROOT).

The `--cache-dir` setting is deprecated in favor of build
time control of the cache location with `--pex-root` and
new support for control of the cache's runtime location with
`--runtime-pex-root` is added. As in the past, the
`PEX_ROOT` environment variable can still be used to
control the cache's runtime location.

Unlike in the past, the [Pex PEX](
https://github.com/pex-tool/pex/releases/download/v2.1.7/pex) we
release can now also be controlled via the `PEX_ROOT` environment
variable. Consult the CLI help for `--no-strip-pex-env`cto find out
more.

* Sanitize PEX_ROOT handling. (929)
* Fix `PEX_*` env stripping and allow turning off. (932)
* Remove second urllib import from compatibility (931)
* Adding `--runtime-pex-root` option. (780)
* Improve interpreter not found error messages. (928)
* Add detail in interpreter selection error message. (927)
* Respect `Requires-Python` in
`PEXEnvironment`. (923)
* Pin our tox version in CI for stability. (924)

* Don't delete the root __init__.py when devendoring. (915)
* Remove unused Interpreter.clear_cache. (911)

* Silence pip warnings about Python 2.7. (908)
* Kill `Pip.spawn_install_wheel` `overwrite` arg. (907)
* Show pex-root from env as default in help output (901)

This release fixes the hermeticity of pip resolver executions when the
resolver is called via the Pex API in an environment with PYTHONPATH
set.

* readme: adding a TOC (900)
* Fix Pex resolver API PYTHONPATH hermeticity. (895)
* Fixup resolve debug rendering. (894)
* Convert `bdist_pex` tests to explicit cmdclass. (897)