Pex

This is a hotfix release for a regression introduced in 2.1.68 for a
narrow class of `--venv` `--no-venv-site-packages-copies` mode PEXes
with special contents on the `PEX_PATH`.

* Fix venv creation for duplicate symlinked dists. (1639)

This release brings a fix for installation of additional data files in
PEX venvs (More on additional data files
[here](https://setuptools.pypa.io/en/latest/deprecated/distutils/setupscript.html?highlight=data_files#installing-additional-files))
as well as a new venv install `--scope` that can be used to create fully
optimized container images with PEXed applications (See how to use this
feature
[here](https://docs.pex-tool.org/recipes.html#pex-app-in-a-container)).

* Support splitting venv creation into deps & srcs. (1634)
* Fix handling of data files when creating venvs. (1632)

This release brings support for `--platform` arguments with a
3-component PYVER portion. This supports working around
`python_full_version` environment marker evaluation failures for
`--platform` resolves by changing, for example, a platform of
`linux_x86_64-cp-38-cp38` to `linux_x86_64-cp-3.8.10-cp38`. This is
likely a simpler way to work around these issues than using the
`--complete-platform` facility introduced in 2.1.66 by 1609.

* Expand `--platform` syntax: support full versions. (1614)

This release brings a new `--complete-platform` Pex CLI option that can
be used instead of `--platform` when more detailed foreign platform
specification is needed to satisfy a resolve (most commonly, when
`python_full_version` environment markers are in-play). This, paired
with the new `pex3 interpreter inspect` command that can be used to
generate complete platform data on the foreign platform machine being
targeted, should allow all foreign platform PEX builds to succeed
exactly as they would if run on that foreign platform as long as
pre-built wheels are available for that foreign platform.

Additionally, PEXes now know how to set a usable process name when the
PEX contains the `setproctitle` distribution. See
[here](https://docs.pex-tool.org/recipes.html#long-running-pex-applications-and-daemons)
for more information.

* Add support for `--complete-platform`. (1609)
* Introduce `pex3 interpreter inspect`. (1607)
* Use setproctitle to sanitize `ps` info. (1605)
* Respect `PEX_ROOT` in `PEXEnvironment.mount`. (1599)

This release really brings support for mac universal2 wheels. The fix
provided by 2.1.64 was partial; universal2 wheels could be resolved at
build time, but not at runtime.

* Upgrade vendored packaging to 20.9. (1591)

This release brings support for mac universal2 wheels.

* Update vendored Pip to 386a54f0. (1589)