[issue36307] Upgrade Travis CI config to Xenial from near-EoL Trusty and remove obsolete sudo: false key

[Inada Naoki]

Change by Inada Naoki <songofacandy at gmail.com>:


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pull_requests: +12360

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36307>
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