Deprecate yield_fixtures

[nicoddemus]

I just realized that we never actually deprecated yield_fixtures, only in docs. I think we should start warning with RemovedInPytest4Warnings so eventually we can turn them into errors in pytest 4.0.

[pytestbot]

GitMate.io thinks possibly related issues are #3661 (deprecate calling fixtures as functions), #3050 (deprecate pytest.config), #1684 (Deprecate "pytest_funcarg__" prefix for fixtures), #3653 (Avoid deprecated attrs feature convert), and #2803 (Deprecate TerminalReporter._tw).

[nicoddemus]

@RonnyPfannschmidt what do you think?