Pex

* Make `tox -evendor` idempotent. (651)
* Fix invalid regex and escape sequences causing DeprecationWarning
(646)
* Follow PEP 425 suggestions on distribution preference. (640)
* Setup interpreter extras in InstallerBase. (635)
* Ensure bootstrap demotion is complete. (634)

* Fix pex force local to handle PEP 420. (613)
* Vendor `setuptools` and `wheel`. (624)

* Fixup PEXEnvironment extras resolution. (617)
* Repair unhandled AttributeError during pex bootstrapping. (599)

This release brings an exit code fix for pexes run via entrypoint as
well as a fix for finding scripts when building pexes from wheels with
dashes in their distribution name.

* Update PyPI default URL to pypi.org (610)
* Pex exits with correct code when using entrypoint (605)
* Fix *_custom_setuptools_usable ITs. (606)
* Update pyenv if neccesary (586)
* Fix script search in wheels. (600)
* Small Docstring Fix (595)

This release brings a fix to handle top-level requirements with
environment markers, fully completing environment marker support.

* Filter top-level requirements against env markers. (592)

This release fixes pexes such that they fully support environment
markers, the canonical use case being a python 2/3 pex that needs to
conditionally load one or more python 2 backport libs when running under
a python 2 interpreter only.

* Revert "Revert "Support environment markers during pex activation.
(582)""