Implicit import of a function in its docstring

[ev-br]

Reported as being used in scikit-image. Not sure if it's that good of an idea, but if there actually is a user request, then why not, maybe with a flag in DTConfig.

This would be likely a use case for DocTest.extraglobs -- will need to populate by in a DTParser.parse (or as post processing, from the test.name attribute)