make doctest-requires directive more global #60
Description
In some cases it would make sense to be able to use doctest-requires (or a new one, e.g. doctest-requires-all) similarly to doctest-skip-all. It would be useful for the case when a dependency is introduced in the middle of the narrative flow of examples and is required from there on. Or when the examples are known to be failing on older python versions (e.g. there are known OrderedDict vs dict differences when the literal comparison makes little sense)