This type is most prominent in `pytest.raises` and we should allow to
refer to it by a public name.
The type is not in a perfectly "exposable" state. In particular:
- The `traceback` property with type `Traceback` which is derived from
the `py.code` API and exposes a bunch more types transitively. This
stuff is *not* exported and probably won't be.
- The `getrepr` method which probably should be private.
But they're already used in the wild so no point in just hiding them
now.
The __init__ API is hidden -- the public API for this are the `from_*`
classmethods.
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