Explain that implements()/implementsOnly()/classProvides() not only
should not be "preferred", but are entirely not functional on Python 3.

(These notes should probably use some Sphinx directive to stand out
better, but I'm not well versed in Sphinx and I don't know which one I
should use.)

YAML borrowed from zopefoundation/persistent.

Always build manylinux wheels

This PR makes it so we _always_ build manylinux wheels, but only upload
them for git tags.

Highlights:

- Move regular Linux Python builds outside the build matrix, for
  conciseness.

- Switch manylinux1 builds to manylinux2010 (they should still get the
  manylinux1 tag if they're compatible, but they'll be built with a more
  modern compiler toolchain).

- Move twine upload from .manylinux.sh into .travis.yml's after_success,
  right next to the twine upload used for Mac OS builds.

- This means we can't short-circuit the builds with 'exit 0' in
  before_install:, so overwrite the install: and script: sections too.

- Move the 'docker pull' from .manylinux.sh into the install: section
  so it's not empty.

- Provide twine credentials via environment variables instead of
  command-line/pypirc file.

- Put related commands inside a single multiline if statement.

- Use setup.py -q test (quiet build) instead of setup.py test -q (which
  does nothing at all AFAICT).

This mirrors changes made to other zopefoundation packages tracked in
https://github.com/zopefoundation/meta/issues/11.

Add support for Python 3.8