Change shebangs to `#!/usr/bin/env python` instead of
`#!/usr/bin/env python2.7`.

jobset.run was changed on 16 October 2015 in e4c35610
to return the number of failures and the result set. On 20 October 2016
in 5c691c63 jobset.run was changed to
allow jobs to be skipped, but the introduced code path only returned
the result set, giving jobset.run inconsistent return value semantics.
On 24 October 2016 7e9bd6ca introduced
a use of jobset.run's "skip_jobs" return value semantics, and on 8
March 2017 d159af31 resolved
jobset.run's inconsistent return value semantics but failed to correct
run_tests_matrix's use of the inconsistent "skip_jobs" return value
semantics.

Update run_tests/*.py to use six based isomorphisms and print function
from __future__ module.

Allow Node tests to run with or without UV, change default version to 7, add some portability tests. Also make some more core tests work with libuv