Use | operator for Union traits

Following [PEP 604](https://www.python.org/dev/peps/pep-0604/) which allows constructs like `int | float` as a replacement for `Union[int, float]` should we allow `Int | Float` as an (optional) replacement for `Union(Int, Float)`?

The good:
- meaning is fairly clear
- alignment with Python typing

The bad:
- if you want metadata then you'd have to do something like `(Int | Float)(foo='bar')` which is not very clear compared to `Union(Int, Float, foo='bar')`

The ugly:
- `Int | Float` or `Int() | Float()`? It may have to be the latter, but presumably Python is going to grow some hook for `|` for types with this change, the same way it did for getitem.