empty parameterset - transtion to failure by default

[RonnyPfannschmidt]

followup to #3970

when the empty parameterset idea was introduced, it was pretty bad, it still is and should actually turn bac kto a failure by default

however to facilitate that we need

• a transition plan with a warning
• a sensible mechanism to override the global option per parametrize, so people an make local informed choices

[nicoddemus]

Thanks for creating this follow up @RonnyPfannschmidt.

I think we should emit the warning if we encounter an empty parameter set and the empty_parameter_set_mark is not set, warning that the default value will change and users should update their pytest.ini file if they like the skip behavior.

After two releases we just change the defaults, people will have seen the warning now and update their settings if they care.

Not sure we need to add the ability to change the ini setting from each parametrize though, as we don't have that option now. But if we want to add it should be simple, an option to parametrize which can be used to override the ini setting: skip, xfail and fail_at_parametrize, unless if I'm missing something.