Pex

This release fixes a concurrency bug in the `pex --lock ...` artifact
downloading.

* Fix `pex --lock ...` concurrent download errors. (1854)

This releases fixes regressions in foreign `--platform` handling and
artifact downloading introduced by 1787 in Pex 2.1.91 and 1811 in
2.1.93.

In addition, PEXes can now be used as `sys.path` entries. Once on the
`sys.path`, via `PYTHONPATH` or other means, the code in the PEX can be
made importable by first importing `__pex__` either as its own
stand-alone import statement; e.g.: `import __pex__; import psutil` or
as a prefix of the code to import from the PEX; e.g.:
`from __pex__ import psutil`.

* Tags should be patched for `--platform`. (1846)
* Add support for importing from PEXes. (1845)
* Fix artifact downloads for foreign platforms. 1851

This release patches a hole left by 1828 in the Pex 2.1.95 release
whereby, although you could run a PEX under a too-long PEX_ROOT you
could not build a PEX under a tool-long PEX_ROOT.

* Avoid ENOEXEC for Pex internal `--venv`s. (1843)

This is a hotfix release that fixes `--venv` mode `PEX_EXTRA_SYS_PATH`
propagation introduced in Pex 2.1.95 to only apply to `sys.executable`
and not other Pythons.

* Fix `--venv` `PEX PEX_EXTRA_SYS_PATH` propagation. (1837)

This release brings two new `pex3 lock` features for `--style universal`
locks.

By default, universal locks are created to target all operating systems.
This can cause problems when you only target a subset of operating
systems and a lock transitive dependency that is conditional on an OS
you do not target is not lockable. The new
`--target-system {linux,mac,windows}` option allows you to restrict the
set of targeted OSes to work around this sort of issue. Since PEX files
currently only support running on Linux and Mac, specifying
`--target-system linux --target-system mac` is a safe way to
pre-emptively avoid these sorts of locking issues when creating a
universal lock.

Previously you could not specify the `--platform`s or
`--complete-platform`s you would be using later to build PEXes with when
creating a universal lock. You now can, and Pex will verify the
universal lock can support all the specified platforms.

As is usual there are also several bug fixes including properly
propagating `PEX_EXTRA_SYS_PATH` additions to forked Python processes,
fixing `pex3 lock export` to only attempt to export for the selected
target and avoiding too long shebang errors for `--venv` mode PEXes in a
robust way.

* Fix `PEX_EXTRA_SYS_PATH` propagation. (1832)
* Fix `pex3 lock export`: re-use `--lock` resolver. (1831)
* Avoid ENOEXEC for `--venv` shebangs. (1828)
* Check lock can resolve platforms at creation time. (1824)
* Support restricting universal lock target os. (1823)

This is a hotfix release that fixes a regression introduced in Pex
2.1.93 downloading certain sdists when using `pex --lock ...`.

* Fix `pex --lock ...` handling of sdists. (1818)