repair broken tests in master

[kwlzn]

when #461 broke travis status, a handful of test failures unfortunately leaked into master.

this ticket tracks repairing those failures to get master green again.

this travis run should reveal the breakage beyond #461: https://travis-ci.org/pantsbuild/pex/builds/377906298

[kwlzn]

lint fixes out here: #464

[kwlzn]

isort fixes out here: #465

[kwlzn]

a round of @CMLivingston fixes out here: #466

[kwlzn]

there are still at least two outstanding classes of issues that I can see in master.

1. https://travis-ci.org/pantsbuild/pex/jobs/377947612#L434

2. https://travis-ci.org/pantsbuild/pex/jobs/377947612#L244

[kwlzn]

potential fix for #1 out here: #468

[kwlzn]

#1 is fixed and I've added improved traceback logging for the pex integration testing to reveal a full traceback for #2, which shows:

https://travis-ci.org/pantsbuild/pex/jobs/378030033#L322

this seems to relate to locating setuptools for injection into the pex bootstrap payload, but only repros in CI.

[kwlzn]

cut #470 for the two straggler integration tests and will skip those for now.

[kwlzn]

@CMLivingston cut #474 to close #470

there's still some extreme flakiness in the pypy shard, will look into resolving that today.

[kwlzn]

#477 for the pypy issue and #478 for temporary relief. this should get master fully stable again.