Pex

* Fixup pex re-exec during bootstrap. (741)
* Fix resolution of `setup.py` project extras. (739)
* Tighten up namespace declaration logic. (732)
* Fixup import sorting. (731)

We now support reproducible builds when creating a pex via `pex -o
foo.pex`, meaning that if you were to run the command again
with the same inputs, the two generated pexes would be byte-for-byte
identical. To enable reproducible builds when building a pex, use the
flags `--no-use-system-time --no-compile`, which will use
a deterministic timestamp and not include `.pyc` files in
the Pex.

In Pex 1.7.0, we will default to reproducible builds.

* add delayed pkg_resources import fix from 713, with an
integration test (730)
* Fix reproducible builds sdist test by properly requiring building
the wheel (727)
* Fix reproducible build test improperly using the -c flag and add a
new test for -c flag (725)
* Fix PexInfo requirements using a non-deterministic data structure
(723)
* Add new `--no-use-system-time` flag to use a
deterministic timestamp in built PEX (722)
* Add timeout when using requests. (726)
* Refactor reproducible build tests to assert that the original pex
command succeeded (724)
* Introduce new `--no-compile` flag to not include .pyc
in built pex due to its non-determinism (718)
* Document how Pex developers can run specific tests and run Pex from
source (720)
* Remove unused bdist_pex.py helper function (719)
* Add failing acceptance tests for reproducible Pex builds (717)
* Make a copy of globals() before updating it. (715)
* Make sure `PexInfo` is isolated from
`os.environ`. (711)
* Fix import sorting. (712)
* When iterating over Zipfiles, always use the Unix file separator to
fix a Windows issue (638)
* Fix pex file looses the executable permissions of binary files
(703)

This is the first release including only a single PEX pex, which
supports execution under all interpreters pex supports.

* Fix pex bootstrap interpreter selection. (701)
* Switch releases to a single multi-pex. (698)

This release fixes long-broken resolution of abi3 wheels.

* Use all compatible versions when calculating tags. (692)

This release un-breaks [lambdex](https://github.com/wickman/lambdex).

* Restore `pex.pex_bootstrapper.is_compressed` API. (685)
* Add the version of pex used to build a pex to build_properties.
(687)
* Honor interpreter constraints even when PEX_PYTHON and
PEX_PYTHON_PATH not set (668)

This release changes the behavior of the `--interpreter-constraint`
option. Previously, interpreter constraints were ANDed, which made it
impossible to express constraints like '>=2.7,<3' OR '>=3.6,<4';
ie: either python 2.7 or else any python 3 release at or above 3.6. Now
interpreter constraints are ORed, which is likely a breaking change if
you have scripts that pass multiple interpreter constraints. To
transition, use the native `,` AND operator in your constraint
expression, as used in the example above.

* Provide control over pex warning behavior. (680)
* OR interpreter constraints when multiple given (678)
* Pin isort version in CI (679)
* Honor PEX_IGNORE_RCFILES in to_python_interpreter() (673)
* Make `run_pex_command` more robust. (670)