Pex

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)""

This is a hotfix release for 1.4.8 that fixes a regression in
interpreter setup that could lead to resolved distributions failing to
build or install.

* Cleanup `PexInfo` and `PythonInterpreter`.
(581)
* Fix resolve regressions introduced by the 1.4.8. (580)
* Narrow the env marker test. (578)
* Documentation for 569 (574)

This release adds support for `-c` and `-m`
PEX file runtime options that emulate the behavior of the same arguments
to `python` as well a fix for handling the non-standard
platform reported by setuptools for Apple system interpreters in
addition to several other bug fixes.

* Fix PEXBuilder.clone. (575)
* Fix PEXEnvironment platform determination. (568)
* Apply more pinning to jupyter in IT. (573)
* Minimize interpreter bootstrapping in tests. (571)
* Introduce 3.7 to CI and release. (567)
* Add OSX shards. (565)
* Add support for `-m` and `-c` in interpreter
mode. (563)
* Ignore concurrent-rename failures. (558)
* Fixup test_jupyter_appnope_env_markers. (562)

This is a hotfix release for a regression in setuptools compatibility
introduced by 542.

* Fixup `PEX.demote_bootstrap`: fully unimport. (554)

This release opens up setuptools support for more modern versions that
support breaking changes in `setup` used in the wild.

* Fix for super() usage on "old style class" ZipFile (546)
* Cleanup bootstrap dependencies before handoff. (542)
* Support -c for plat spec dists in multiplat pexes. (545)
* Support `-` when running as an interpreter. (543)
* Expand the range of supported setuptools. (541)
* Preserve perms of files copied to pex chroots. (540)
* Add more badges to README. (535)
* Fixup CHANGES PR links for 1.4.5.

This release adds support for validating pex entrypoints at build time
in addition to several bugfixes.

* Fix PEX environment setup. (531)
* Fix installers to be insensitive to extras iteration order. (532)
* Validate entry point at build time (521)
* Fix pex extraction perms. (528)
* Simplify `.travis.yml`. (524)
* Fix `PythonInterpreter` caching and ergonomics. (518)
* Add missing git dep. (519)
* Introduce a controlled env for pex testing. (517)
* Bump wheel version to latest. (515)
* Invoke test runner at a more granular level for pypy shard. (513)