Pex

This release fixes a performance regression in which pip would
re-tokenize `--find-links` pages unnecessarily. The parsed pages are now
cached in a pip patch that has also been submitted upstream.

* Re-vendor pip (890)
* Add a clear_cache() method to PythonInterpreter. (885)
* Error eagerly if an interpreter binary doesn't exist. (886)

This release fixes a bug in which interpreter discovery failed when
running from a zipped pex.

* Use pkg_resources when isolating a pex code chroot. (881)

This release significantly improves performance and correctness of
interpreter discovery, particularly when pyenv is involved. It also
provides a workaround for EPERM issues when hard linking across devices,
by falling back to copying. Resolve error checking also now accounts for
environment markers.

* Revert "Fix the resolve check in the presence of platform
constraints. (877)" (879)
* [resolver] Fix issue with wheel when using `--index-url` option
(865)
* Fix the resolve check in the presence of platform constraints.
(877)
* Check expected pex invocation failure reason in tests. (874)
* Improve hermeticity of vendoring. (873)
* Temporarily skip a couple of tests, to get CI green. (876)
* Respect env markers when checking resolves. (861)
* Ensure Pex PEX constraints match pex wheel / sdist. (863)
* Delete unused pex/package.py. (862)
* Introduce an interpreter cache. (856)
* Re-enable pyenv interpreter tests under pypy. (859)
* Harden PythonInterpreter against pyenv shims. (860)
* Parallelize interpreter discovery. (842)
* Explain hard link EPERM copy fallback. (855)
* Handle EPERM when Linking (852)
* Pin transitive dependencies of vendored code. (854)
* Kill empty setup.py. (849)
* Fix `tox -epackage` to create pex supporting 3.8. (843)
* Fix Pex to handle empty ns package metadata. (841)

replaces and improves upon Pex 1-series functionality.

* Fix pex resolving for foreign platforms. (835)
* Use pypa/packaging. (831)
* Upgrade vendored setuptools to 42.0.2. (832)
* De-vendor pex just once per version. (833)
* Support VCS urls for vendoring. (834)
* Support python 3.8 in CI. (829)
* Fix pex resolution to respect `--ignore-errors`. (828)
* Kill `pkg_resources` finders monkey-patching. (827)
* Use flit to distribute pex. (826)
* Cleanup extras_require. (825)

This release fixes a regression in handling explicitly requested
`--index` or `--find-links` http (insecure) repos. In addition,
performance of the pex 2.x resolver is brought in line with the 1.x
resolver in all cases and improved in most cases.

* Unify PEX build-time and runtime wheel caches. (821)
* Parallelize resolve. (819)
* Use the resolve cache to skip installs. (815)
* Implicitly trust explicitly requested repos. (813)

This is a hotfix release that fixes a bug exposed when Pex was asked to
use an interpreter with a non-canonical path as well as fixes for
'current' platform handling in the resolver API.

* Fix current platform handling. (801)
* Add a test of pypi index rendering. (799)
* Fix `iter_compatible_interpreters` path biasing. (798)