Adjust stacklevel for compat warnings

[nicoddemus]

Follow up from #4131 (comment).

Currently the warnings show this:

c:\projects\pytest\.tox\py36\lib\site-packages\_pytest\compat.py:321: RemovedInPytest4Warning: usage of Session.Class is deprecated, please use pytest.Class instead
360  return getattr(object, name, default)
361

We need to adjust the warnings stacklevel.

[pecey]

I can try and fix this. I would need some help though. If I understand correctly, we need to adjust the stack level for the warnings thrown my the warnings plugin itself.

[RonnyPfannschmidt]

@pecey unfortunately thats not how it works - the stacklevel is designated for direct usage, here a wrapper is using getattr - so adjusting the stacklevel for that breaks it

we do have a fix to remove the safe_getattr situation by not parsing compat properties as fixture in #4164

[nicoddemus]

@RonnyPfannschmidt is right, the stacklevel is already properly set:

def test(request):
    request.node.session.Class

test-foo.py::test
  C:\pytest\.tmp\test-foo.py:2: RemovedInPytest4Warning: usage of Session.Class is deprecated, please use pytest.Class instead
    request.node.session.Class

We only have to merge #4164 then. 👍