WIP/RFC: remove pytest.python.Instance [ci skip]

[blueyed]

Ref: #4127

[blueyed]

Feel free to comment, but let's go with #4358 first.

[nicoddemus]

Just to note that this is breaking change as pytest.Instance is part of the API. We should figure out a way to deprecate its access somehow if we are to delete it in the future.

[RonnyPfannschmidt]

@nicoddemus this is one of the changes where i want to do a breaking release without the ramp ups

[nicoddemus]

Without the ramp ups because there's no sane way to generate a warning for this, correct? I agree, I can't think of a portable way to issue that warning either.

[RonnyPfannschmidt]

@nicoddemus correct