`Parser` is used by many plugins and custom hooks. `OptionGroup` is
exposed by the `parser.addgroup` API.
The constructors of both are marked private, they are not meant to be
constructed directly.
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.
The type cannot be constructed directly, but is exported for use in type
annotations, since it is reachable through existing public API.
This also documents `from_call` as public, because at least
pytest-forked uses it, so we must treat it as public already anyway.
Ran Benita