We want to export `pytest.MonkeyPatch` for the purpose of
type-annotating the `monkeypatch` fixture. For other fixtures we export
in this way, we also make direct construction of them (e.g.
`MonkeyPatch()`) private. But unlike the others, `MonkeyPatch` is also
widely used directly already, mostly because the `monkeypatch` fixture
only works in `function` scope (issue #363), but also in other cases. So
making it private will be annoying and we don't offer a decent
replacement yet.
So, let's just make direct construction public & documented.
Ran Benita