Pex

A patch release to fix an issue with the
`--use-first-matching-interpreter` flag.

* Fix `--use-first-matching-interpreter` at runtime. (1014)

This release adds the `--use-first-matching-interpreter` flag, which can
speed up performance when building a Pex at the expense of being
compatible with fewer interpreters at runtime.

* Add `--use-first-matching-interpreter`. (1008)
* Autoformat with Black. (1006)

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)