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

[STINNER Victor]

STINNER Victor <vstinner at redhat.com> added the comment:

INADA-san: I would prefer to use the same OS to test Python 2.7 and 3.7. Are you ok to backport this change? (if it didn't break the CI yet? :-))

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