Also Raise a `KeyError` when attempting to delete a nonexistent key from the extension registry.

* 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.

In the past, most of the config was defined using globals. Today all of 
the config is held on the class instance. Therefore, the `md_globals` 
parameter is no longer necessary.

Fixes #435.

In the past Markdown used to pass extension config settings to the
Extension class via a positional argument named `config`. That was
deprecated in 2.6 in favor of using keyword arguments (`**kwargs`).
Support has been completely dropped. Only keyword arguments are
accepted.

Also checked for `None` so the existing extensions will at least still work.
Of course, that code all gets deleted with the next release and things will
break if extension authors do not update their code. Hope they test there
code with each release and check for warnings.

Also added a note to the release notes.

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.

The builtin extensions will no longer get special treatment and have
the path ("markdown.extensions.") appended . The same applies for
"mdx_" extensions. All names extension must provide the full path.
Fixes #336.

Also deprecating support for passing in extension config settings
as part of the string name. The extension_configs keyword should
be used instead. Fixes #335.

Also raising PendingDeprecationWarnings for positional args or the
"config" keyword on the Extension Class. Pass each setting as a
seperate keyword instead.

Docs and tests are updated. Still need to update extension API docs.

Fixes #325. All extensions can now accept a dict of configs or
**kwargs, not just a list of tuples. Third party extensions may want
to follow suite. Extensions may only accept keyword arguments
in the future. These changes still need to be documented.

A couple things of note:

The CodeHilite extension previously issued a DeprecationWarning
if the old config key `force_linenos` was used. With thins change,
a KeyError will now be raised.

The `markdown.util.parseBoolValue` function gained a new argument:
`preserve_none` (defaults to False), which when set to True, will
pass None through unaltered (will not convert it to False).

As pointed out in #325, setting up Extension configs is kind of a
mess. Some places pass a list of tuples on initialization, others
a dict. And sometimes they're passed as an arg, othertimes a kwarg.
Addiitonaly, the docs are just as inconsistant.

This refactor addresses all those sinerios with tests included.
The existing extensions still need refactored. But the fact that
their tests still pass means we havn't broken third party extensions
either.

This refactor also introduces a new API, which is the prefered
method going forward. All docs should be updated to match.
Whereas previously one might do:

```python
MyExtension(configs={'key': 'foo', 'otherkey': 'bar'})
```

This can now be done:

```python
MyExtension(key='foo', otherkey='bar')
```

Of course, the old way still works for backward compatability.
But that means the `configs` keyword has special status and cannot
be used for another purpose.