Pex

The focus of this release is better support of the `--platform` CLI arg.
Platforms are now better documented and can optionally be resolved to
local interpreters when possible via `--resolve-local-platforms` to
better support creation of multiplatform PEXes.

* Add support for resolving `--platform` locally. (1000)
* Improve `--platform` help. (1002)
* Improve and fix `--platform` help. (1001)
* Ensure pip download dir is uncontended. (998)

A patch release to deploy the PEX_EXTRA_SYS_PATH feature.

* A PEX_EXTRA_SYS_PATH runtime variable. (989)
* Fix typos (986)
* Update link to avoid a redirect (982)

A patch release to fix a symlink issue in remote execution environments.

* use relative paths within wheel cache (979)
* Fix Tox not finding Python 3.8 on OSX. (976)

This release focuses on the resolver API and resolution performance. Pex
2 resolving using Pip is now at least at performance parity with Pex 1
in all studied cases and most often is 5% to 10% faster.

As part of the resolution performance work, Pip networking configuration
is now exposed via Pex CLI options and the `NetworkConfiguration` API
type / new `resolver.resolve` API parameter.

With network configuration now wired up, the `PEX_HTTP_RETRIES` and
`PEX_HTTP_TIMEOUT` env var support in Pex 1 that was never wired into
Pex 2 is now dropped in favor of passing `--retries` and `--timeout` via
the CLI (See: 94)

* Expose Pip network configuration. (974)
* Restore handling for bad wheel filenames to `.can_add()` (973)
* Fix wheel filename parsing in PEXEnvironment.can_add (965)
* Split Pex resolve API. (970)
* Add a `--local` mode for packaging the Pex PEX. (971)
* Constrain the virtualenv version used by tox. (968)
* Improve Pex packaging. (961)
* Make the interpreter cache deterministic. (960)
* Fix deprecation warning for `rU` mode (956)
* Fix runtime resolve error message generation. (955)
* Kill dead code. (954)

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)