Closes#7480.
This allows us to more easily follow our deprecation policy of turning
warnings into errors for the X.0 releases before complete removal in
X.1.
It also makes the deprecation timeline clear to both the users and
pytest developers -- it can be hard to keep track.
Note that the designation is not meant to be a binding contract - if the
time comes for removal of a specific deprecation but we decide it's too
soon, can just bump it to the next major.
Inspired by Django:
https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/submitting-patches/#deprecating-a-feature
Ran Benita