Pex

This release changes pex requirement resolution behavior. Only stable
requirements are resolved by default now. The previous behavior that
included pre-releases can be retained by passing `--pre` on the pex
command line or passing `allow_prereleases=True` via the API.

* Upgrade dependencies to modern version ranges. (352)
* Add support for controlling prerelease resolution. (350)

* Add dummy flush method for clean interpreter exit with python3.6
(343)

* Implement `--constraints` in pex (335)
* Make sure namespace packages (e.g. virtualenvwrapper) don't break
pex (338)

* Expose a PEX instance's path. (332)
* Check for scripts directory in get_script_from_egg (328)

* Make PEX_PATH unify pex sources, as well as requirements. (329)

* Adjust FileFinder import to work with Python 3.6. (318)
* Kill zipmanifest monkeypatching. (322)
* Bump setuptools range to latest. (323)