Add Python 3.7 tests to nightly cron CI run (#7261)

### Problem
We now claim support for both Python 3.6 and Python 3.7. 

A couple of times, we've been hit by issues specific to Python 3.7. So, we want to have CI to catch these issues.

This will close #7162.

### Solution
Add new shards to run with Py37 during the cron stage every night.

This requires getting Py37 onto the shards. We extend our current use of Pyenv to also install Python 3.7 where relevant. 

Note we cannot get Python 3.7 onto the Centos6 Build Engine shard, as the OpenSSL shard is too outdated. Instead, we simply use Ubuntu Xenial to bootstrap the Py37 PEX. This is okay to do because we do not build any wheels with this shard, and only use it for internal testing. See #7421. 

### Result
Cron job now runs with both Python 2.7 and 3.7. See https://travis-ci.org/pantsbuild/pants/builds/509675034 for what the new nightly run will look like.

Daily CI is the same as before.