Modernize the pex venv script.
Modernize the pex venv script.

This converts from grabbing the old Twitter python pex to grabbing
modern pex to match the version specified in 3rdparty to help keep the
pex venv up to date with the codebase dependencies.

Testing Done:
Locally ran this 2x and observed both the proper version (1.1.0) and
proper use of the cached venv in the second run:
`git clean -fdx build-support && ./build-support/pex --version`

Reviewed at https://reviews.apache.org/r/40220/

protochron/aurora,crashlytics/aurora,apache/aurora,protochron/aurora,protochron/aurora,rdelval/aurora,rdelval/aurora,protochron/aurora,crashlytics/aurora,thinker0/aurora,thinker0/aurora,apache/aurora,rdelval/aurora,crashlytics/aurora,protochron/aurora,crashlytics/aurora,thinker0/aurora,medallia/aurora,rdelval/aurora,thinker0/aurora,medallia/aurora,apache/aurora,rdelval/aurora,crashlytics/aurora,thinker0/aurora,crashlytics/aurora,medallia/aurora,medallia/aurora,thinker0/aurora,apache/aurora,apache/aurora,apache/aurora,medallia/aurora,medallia/aurora,protochron/aurora,rdelval/aurora