Upgrade to pex 1.4.3.

[jsirois]

This hacks around a few issues with the 1.4.x pex API. We concoct a
minimal local Platform to pass to resolve where a local interpreter
is passed to work around pex-tool/pex#511.
We also now consolidate PythonInterpreter construction in production
code to helper that ensures the interpreters we create are bare
(isolated) except for the specific extras we require to work around
pex-tool/pex#510.

Upgrading pex to take advantage of the worked around issues noted above
is tracked by #5922.

Fixes #5906

[kwlzn]

thanks John! lgtm!

[illicitonion]

This broke my pants at HEAD:

pants$ ./pants -q --changed-parent=master lint

FAILURE
Exception caught: (<class 'pex.executor.ExecutableNotFound'>)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_loader.py", line 73, in <module>
    main()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_loader.py", line 69, in main
    PantsLoader.run()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_loader.py", line 65, in run
    cls.load_and_execute(entrypoint)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_loader.py", line 58, in load_and_execute
    entrypoint_main()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_exe.py", line 39, in main
    PantsRunner(exiter, start_time=start_time).run()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/pants_runner.py", line 53, in run
    return runner.run()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/local_pants_runner.py", line 45, in run
    self._run()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/local_pants_runner.py", line 90, in _run
    goal_runner_result = goal_runner.run()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/goal_runner.py", line 244, in run
    result = self._execute_engine()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/bin/goal_runner.py", line 232, in _execute_engine
    result = engine.execute(self._context, self._goals)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/engine/legacy_engine.py", line 26, in execute
    self.attempt(context, goals)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/engine/round_engine.py", line 233, in attempt
    goal_executor.attempt(explain)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/engine/round_engine.py", line 49, in attempt
    task.execute()
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/backend/python/tasks/select_interpreter.py", line 75, in execute
    interpreter = self._get_interpreter(interpreter_path_file)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/backend/python/tasks/select_interpreter.py", line 98, in _get_interpreter
    interpreter = create_bare_interpreter(binary)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/src/python/pants/backend/python/pex_util.py", line 22, in create_bare_interpreter
    interpreter_with_extras = PythonInterpreter.from_binary(binary_path)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/build-support/pants_dev_deps.venv/lib/python2.7/site-packages/pex/interpreter.py", line 375, in from_binary
    cls.CACHE[binary] = cls._from_binary_external(binary, path_extras)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/build-support/pants_dev_deps.venv/lib/python2.7/site-packages/pex/interpreter.py", line 337, in _from_binary_external
    stdout, _ = Executor.execute([binary], env=environ, stdin_payload=_generate_identity_source())
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/build-support/pants_dev_deps.venv/lib/python2.7/site-packages/pex/executor.py", line 99, in execute
    process = cls.open_process(cmd=cmd, env=env, cwd=cwd, **kwargs)
  File "/Users/dwagnerhall/src/github.com/pantsbuild/pants/build-support/pants_dev_deps.venv/lib/python2.7/site-packages/pex/executor.py", line 83, in open_process
    raise cls.ExecutableNotFound(cmd, e)

Exception message: caught OSError(2, "No such file or directory: '/opt/twitter_mde/package/eepython27/2b9f44c3673ea5b18e3c12bf36c35ef93e3d03a547f10d19f45d7097b09a85a1/bin/python2.7\\tCPython-2.7.14'") while trying to execute `['/opt/twitter_mde/package/eepython27/2b9f44c3673ea5b18e3c12bf36c35ef93e3d03a547f10d19f45d7097b09a85a1/bin/python2.7\tCPython-2.7.14']` while trying to execute `['/opt/twitter_mde/package/eepython27/2b9f44c3673ea5b18e3c12bf36c35ef93e3d03a547f10d19f45d7097b09a85a1/bin/python2.7\tCPython-2.7.14']`

[jsirois]

Hrm. The version bump in the task was meant to cover the interpreter file format change. I'll followup here presently. Sorry for the breakage!

[jsirois]

@illicitonion you probably figured this out but the workaround is a ./pants clean-all :(

[jsirois]

Fix in #5925

[illicitonion]

Thanks for the quick fix!