[pyoutline] Layer module cleanup

[anton-ubi]

Summary.
Surface cleanup and typing annotation of the layer module.

Changes

• remove spotted python-2 artifacts (objects, with_metaclass, super with args)
• use super() when possible
• use f-string when meaningful
• add typing annotations when non ambiguous
• explicit a bit more what parameters are actually hidden behind Layer object kwargs
• Correct typo and grammar in docstrings and comments
• Add new LayerType enum to represent Layer types (render, util, post)
• Add Layer string and repr representation

Rational

• Type annotations: The main goal is to enable better auto-completion when outlining layers. Prior to that PR, it is very difficult to know in advance the name and type of the parameters accepted by a Layer object.
• LayerType enum: easier access to the available type values. Same as for typing annotations: we can now benefit of the auto-completion to know which types exist.
• The rest is mostly house-cleaning to keep up, especially since we dropped python2.

Note to reviewers
PR can be reviewed per commit for smaller scoped diffs