* nodes: add a `default` param to Template.get

Similar to dict.get, Template.get with a default param supplied will
return that value instead of raising an exception. If default is unset,
Template.get will keep its previous behavior and raise an exception.

* nodes: Add __getitem__, __setitem__, and __delitem__ to Template

These are just aliases for existing methods, without the ability to
specifiy additional parameters. However, including them makes Template
more dict-like, so it's a good idea to have them.

* nodes: Use def instead of assignment of a lambda in Template

Per PEP8, there is no benefit to using a lambda here, and some
downsides. It's the same number of SLOC either way, so might as well
change it.

Use Github Actions to build macOS wheels too

For 3.5+ on the latest macOS version.

Just like the Windows wheels, these allow for Linux users to install
mwparserfromhell and use the faster CTokenizer without needing to
have build tools installed.

Under the hood, this uses pypa manylinux1 docker image to build and
tag the wheels, then publishes them to pypi if a new tag was pushed.

Fixes #170.

Drop Python 2 support

In addition to replacing the manual version check, this will also
instruct pip to download an older version of mwparserfromhell for
users running earlier Python versions rather than just getting something
broken.

Fixes #221.

move smart_list into sub-package/multiple files

see #198

Step one of refactoring - making SmartList into its own
package, with each class having its own file.  No code
changes were made.

Note that SmartList and ListProxy import each other,
so had to import SmartList as a full package name
rather than use from ... import ... construct.

ignore InteliJ IDEA files

add missing public items to __all__

Added a few more public classes, and sorted the __all__ value.
The Attribute and Parameter classes are not included as they can
be imported from the `mwparserfromhell.nodes.extras` without
linter complaining.  They could be added to this __all__ too.