…s lineno computation (GH-136930)

Previously, DocTest's lineno of functions and methods decorated with
functools.cache(), functools.lru_cache() and functools.cached_property()
was not properly returned (None was returned) because the
computation relied on inspect.isfunction() which does not consider the
decorated result as a function.

We now use the more generic inspect.isroutine(), as elsewhere
in doctest's logic.

Also, added a special case for functools.cached_property().