Travis finally released xenial (16.04).

[OniOni]

Travis released Xenial (16.04) (see https://changelog.travis-ci.com/ubuntu-xenial-16-04-build-environment-is-here!-79690) This PR moves us to this new image.

The two major changes are:

• Switch to Python 3.5 in CI.
  This coincides with prior discussions in Slack that we want to target Python 3.5 as the minimum Python 3 version. This is to give us access to native type hints and async/await syntax, along with general improvements such as fixing the shutil bug identified in Unblacklist JVM check_style Python 3 integration test thanks to using Python 3.5+ #6882.

• Switch to openjdk over oraclejdk. This is mostly because travis only bakes version 10 and 11 of the oracle jdk and has removed all the tooling to get other versions of that jdk. While image comes with version 8 of the openjdk.

[Eric-Arellano]

Thank you Mathieu for pushing this through!

John and Stu, if you look at logs from most recent logs you’ll see how Python now resolves, including Pyenv installing 3.6 and 3.7. (I’m afk, sorry I can’t grab those lines directly)

[stuhood]

Thanks, looks good. Would figure out whether the JAVA_HOME entry is actually necessary, and would wait for @jsirois to sanity check whether build-support/bin/travis-ci.sh is still resulting in relevant python(s).

[stuhood]

There is a stray k on the end here, which makes me wonder whether this JAVA_HOME entry is actually being used.

[OniOni]

That is a very good point. I had removed it, believing it was not needed, but it was causing failures. I'll give it a spin without the JAVA_HOME, for sanity's sake.

[OniOni]

Yeah, absence of JAVA_HOME causes a lot of failures.

[jsirois]

The log output does show relevant pythons, and travis-ci.sh concocts this output by actually invoking the interpreters in question:

pants/build-support/bin/travis-ci.sh

Lines 21 to 31 in dd15f41

	function list_pythons {
	which -a python{,2,3} | sort -u | while read python_bin; do
	${python_bin} <<EOF
	import os
	import sys
	print('{} -> {}'.format(os.path.realpath(sys.executable),
	'.'.join(map(str, sys.version_info[:3]))))
	EOF
	done
	}

Picking a random shard I read:

Visible pythons are now:
/home/travis/virtualenv/python2.7.15/bin/python -> 2.7.15
/opt/python/2.7.15/bin/python2.7 -> 2.7.15
/opt/python/3.6.7/bin/python3.6 -> 3.6.7
/opt/python/3.7.1/bin/python3.7 -> 3.7.1
/usr/bin/python2.7 -> 2.7.12
/usr/bin/python3.5 -> 3.5.2

LGTM

We may no longer need the travis-ci.sh wrapper, but that can be for another change.