Pex

This release brings a slew of small fixes across the code base.

When creating locks for foreign platforms,
`pex3 lock {create,update,sync}` now allows locking sdists that use
PEP-517 build backends that do not support the
`prepare_metadata_for_build_wheel` hook and whose product is a wheel not
compatible with the foreign platform. This is decidedly a corner case,
but one encountered with the `mesonpy` build backend which seems to have
traction in the scientific computing world in particular.

The recent re-vamp of the PEX REPL is now fixed to respect common
conventions for controlling terminal output via the `NO_COLOR`,
`FORCE_COLOR` and `TERM` environment variables.

The examples in the [buildingpex docs](
https://docs.pex-tool.org/buildingpex.html) had bit-rotted. They have
been refreshed and now all work.

Finally, both the Pex CLI and PEX files support the ambient OS standards
for user cache directories. Instead of using `~/.pex` as the default
`PEX_ROOT` cache location, the default is now `~/.cache/pex` on Linux (
but respecting `XDG_CACHE_HOME` when set) and `~/Library/Caches/pex` on
Mac.

* Lock sdists in more cases for foreign platforms. (2508)
* Respect `NO_COLOR`, `FORCE_COLOR` & `TERM=dumb`. (2507)
* Fix `buildingpex.rst` examples. (2506)
* Respect OS user cache location conventions. (2505)

This release fixes the PEX repl for [Python Standalone Builds][PBS]
Linux CPython PEX scies. These PEXes ship using a version of libedit
for readline support that does not support naive use of ansi terminal
escape sequences for prompt colorization.

* Fix PEX repl prompt for Linux PBS libedit. (2503)

This release adds support for `--venv-system-site-packages` when
creating a `--venv` PEX and `--system-site-packages` when creating a
venv using the `pex-tools` / `PEX_TOOLS=1` `venv` command or when using
the `pex3 venv create` command. Although this breaks PEX hermeticity, it
can be the most efficient way to ship partial PEX venvs created with
`--exclude`s to machines that have the excluded dependencies already
installed in the site packages of a compatible system interpreter.

* Support `--system-site-packages` when creating venvs. (2500)

This release enhances the REPL your PEX drops into when it either
doesn't have an entry point or you force interpreter mode with the
`PEX_INTERPRETER` environment variable. There is now clear indication
you are running in a PEX hermetic environment and a `pex` command
added to the REPL that you can use to find out more details about the
current PEX environment.

* Add PEX info to the PEX repl. (2496)

This release fixes `--inject-env` when used in combination with a
`--scie-busybox` so that the injected environment variable can be
overridden at runtime like it can form a traditional PEX.

In addition, running a PEX with the Python interpreter `-i` flag or
`PYTHONINSPECT=x` in the environment causes the PEX to enter the
Python REPL after evaluating the entry point, if any.

* Allow `--inject-env` overrides for `--scie-busybox`. (2490)
* Fix PEXes for `-i` / `PYTHONINSPECT=x`. (2491)

This release brings support for creating PEX scies for PEXes targeting
[PyPy][PyPy]. In addition, for PEX scies targeting CPython, you can now
specify `--scie-pbs-stripped` to select a stripped version of the
[Python Standalone Builds][PBS] CPython distribution embedded in your
scie to save transfer bandwidth and disk space at the cost of losing
Python debug symbols.

Finally, support is added for `--scie-busybox` to turn your PEX into a
multi-entrypoint [BusyBox][BusyBox]-like scie. This support is
documented in depth at https://docs.pex-tool.org/scie.html

* Support `--scie` for PyPy & support stripped CPython. (2488)
* Add support for `--scie-busybox`. (2468)

[PyPy]: https://pypy.org/
[BusyBox]: https://www.busybox.net/