twistedchecker reports missing documentation when the documentation comes from an interface

[Richard Wall]

Often the api documentation for a class, method or function comes from a zope.interface in which case the docstring can be omitted.

twistedchecker should handle this situation.

See:
 * https://buildbot.twistedmatrix.com/builders/twistedchecker/builds/244/steps/run-twistedchecker/logs/new%20twistedchecker%20errors

{{{
************* Module twisted.names.common
W9208: 66,4:ResolverBase.query: Missing docstring
W9208: 80,4:ResolverBase.lookupAddress: Missing docstring
W9208: 84,4:ResolverBase.lookupIPV6Address: Missing docstring
W9208: 88,4:ResolverBase.lookupAddress6: Missing docstring
W9208: 92,4:ResolverBase.lookupMailExchange: Missing docstring
}}}

 * https://twistedmatrix.com/trac/ticket/4685

[Richard Wall (richardw)]

I wrote a very hacky first attempt in lp:~richardw/twistedchecker/inherited-interface-documentation-1132540

I guess it needs refactoring, testing and possibly reworking so that it doesn't actually import the interface classes.

[Richard Wall (richardw)]

Tom Prince did an initial code review and I have since improved the branch so that it no longer imports any modules.

If someone has time to look at it I'd appreciate another code review. See the review request above.