* Add missing type hints (these are intended as an initial set of type hints, to be added upon and improved later)
* Setup MyPy to run as a Github Action
* Add ChainableUndefined allowing getattr & getitem
Allows using default values with chains of items or attributes that may
contain undefined values without raising a jinja2.exceptions.UndefinedError.
>>> import jinja2
>>> env = jinja2.Environment(undefined=jinja2.ChainableUndefined)
>>> env.from_string("{{ foo.bar['baz'] | default('val') }}").render()
'val'
* Remove class decorator from ChainableUndefined
