Pex

This release fixes a deadlock when building PEXes in parallel via the
new `--lock` flag.

* Avoid deadlock error when run in parallel. (1694)

This release fixes multiplatform `--lock` resolves for sdists that are
built to multiple platform specific wheels, and it also introduces
support for VCS requirements in locks.

* Add support for locking VCS requirements. (1687)
* Fix `--lock` for multiplatform via sdists. (1689)

This is a hotfix for various PEX issues:

This release fixes an old bug with `--venv` PEXes initially executed
with either `PEX_MODULE` or `PEX_SCRIPT` active in the environment.

* Fix venv creation to ignore ambient PEX env vars. (1669)

This release fixes the instability introduced in 2.1.68 by switching to
a more robust means of determining venv layouts. Along the way it
upgrades Pex internals to cache all artifacts with strong hashes (
previously sha1 was used). It's strongly recommended to upgrade or use
the exclude `!=2.1.68,!=2.1.69,!=2.1.70` when depending on an open-ended
Pex version range.

* Switch Pex installed wheels to `--prefix` scheme. (1661)

This is another hotfix release for 2.1.68 that fixes a bug in `*.data/*`
file handling for installed wheels which is outlined in [PEP
427](https://www.python.org/dev/peps/pep-0427/#installing-a-wheel-distribution-1-0-py32-none-any-whl)

* Handle `*.data/*` RECORD entries not existing. (1644)