cElementTree is a deprecated alias for ElementTree since Python 3.3.

Also drop the recommendation to import etree from markdown.util,
and deprecate markdown.util.etree.

* Python syntax upgraded using `pyupgrade --py3-plus`
* Travis no longer uses `sudo`. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

See #760 for Python Version Support Timeline and related dicussion.

Serializer should only escape & in attributes if not part of & 

Better regex avoid Unicode and `_` in amp detection.

In general, we don't want to escape already escaped content, but with code content, we want literal representations of escaped content, so have code content explicitly escape its content before placing in AtomicStrings.

Closes #669.

Because of the missing comma, the last two tags in the list, `meta` and `param` are interpreted as a single string literal `"metaparam"`, so the serializer fails to correctly produce empty elements for `<meta>` and `<param>` tags.

Should be 100% coverage now. The ProcessingInstruction needed to be
imported directly from ElementTree as PY27 was using a PIProxy which
resulted in a  bug. Interestingly, PY3 worked fine.

Also removed the encoding code as it was not used. Besides it was only
ever accessable from a private function.

Fixes #679.

Got all but a couple files in the tests (ran out of time today).
Apparently I have been using some bad form for years (although a few
things seemed to look better before the update). Anyway, conformant now.