[Checkins] [zopefoundation/persistent] cc436e: Travis builds: always build manylinux wheels

[Marius Gedminas]

  Branch: refs/heads/fix-wheels
  Home:   https://github.com/zopefoundation/persistent
  Commit: cc436e291e37c70767a9a811e3e258c8e9636cc7
      https://github.com/zopefoundation/persistent/commit/cc436e291e37c70767a9a811e3e258c8e9636cc7
  Author: Marius Gedminas <marius at gedmin.as>
  Date:   2019-11-08 (Fri, 08 Nov 2019)

  Changed paths:
    M .manylinux.sh
    M .travis.yml

  Log Message:
  -----------
  Travis builds: always build manylinux wheels

This also reformats the YAML, for which I apologize, but it was hard to
work on it without doing that.

It drops dist: xenial which has become the default.

It renames matrix: to jobs: because that's the preferres spelling
according to Travis CI docs.

It adds names to all the special build jobs.

It moves regular Linux Python builds outside build matrix, for
conciseness (unfortunately this means 3.6 builds are now done twice:
once without -fwrapv and once with)

It moves docker pull and twine upload out of .manylinux.sh and into
.travis.yml.

It unifies twine upload calls by supplifying username/password via
environment variables.

The changes largely mirror those in
https://github.com/zopefoundation/BTrees/pull/115.