Allow pex 0.8.x to parse older pex files.  Restrict to use of older setuptools.

Allow pex 0.8.x to parse older pex files.  Restrict to use of setuptools
prior to 8.x which changes to PEP440 package versioning (and setuptools 9.x
which changes pkg_resources to a package.)

Testing Done:
$ tox

Reviewed at https://rbcommons.com/s/twitter/r/1551/