Add a runtime check for exported classes which should not be subclassed

From @bluetech: we expose some classes in our API which are not meant for subclassing. We indicate this with @final annotations but these are only effective when type-checking. The following is a suggestion how to add a runtime check.

it's quite clean to add a runtime deprecation warning, so might be worth doing in this PR:

def __init_subclass__(cls, **kwargs):
    warnings.warn(f"subclassing {cls} is deprecated", PytestDeprecationWarning)

def runtime_final(cls: T) -> T:
    cls.__init_subclass__ = __init_subclass__  # type: ignore
    return cls


...
@final
@runtime_final
class Ham:
    pass

Originally posted by @graingert in https://github.com/pytest-dev/pytest/pull/8017#discussion_r520805087

The issue names no implementation file or test; begin by reviewing the exported @final classes and the referenced pytest-dev/pytest#8017 discussion. Done means a runtime check is defined for subclassing classes intended not to be subclassed, with tests covering the resulting behavior.