Pex

This release fixes spurious collision warnings & errors when building
venvs from PEXes that contain multiple distributions contributing to the
same namespace package.

* Allow for duplicate files in venv population. (1572)

This release exposes three Pip options as Pex options to allow building
PEXes for more of the Python distribution ecosystem:

1. `--prefer-binary`: To prefer older wheels to newer sdists in a
resolve which can help avoid problematic builds.
2. `--[no]-use-pep517`: To control how sdists are built: always using
PEP-517, always using setup.py or the default, always using
whichever is appropriate.
3. `--no-build-isolation`: To allow distributions installed in the
environment to be seen during builds of sdists. This allows working
around distributions with undeclared build dependencies by
pre-installing them in the environment before running Pex.

* Expose more Pip options. (1561)

This release fixes a regression in Pex `--venv` mode compatibility with
distributions that are members of a namespace package that was
introduced by 1532 in the 2.1.57 release.

* Merge packages for `--venv-site-packages-copies`. (1557)

This release fixes a bug that prevented creating PEXes when duplicate
compatible requirements were specified using the pip-2020-resolver.

* Fix Pex to be duplicate requirement agnostic. (1551)

This release adds the boolean option `--venv-site-packages-copies` to
control whether `--venv` execution mode PEXes create their venv with
copies (hardlinks when possible) or symlinks. It also fixes a bug that
prevented Python 3.10 interpreters from being discovered when
`--interpreter-constraint` was used.

* Add knob for `--venv` site-packages symlinking. (1543)
* Fix Pex to identify Python 3.10 interpreters. (1545)

This release fixes a bug handling relative `--cert` paths.

* Always pass absolute cert path to Pip. (1538)