Modify twisted.protocols.sip to use twisted.cred

New submission from kelly <None>:

Attached patch modifies twisted.protocols.sip to use twisted.cred internally rather than the deprecated classes in the sip module.

Had a problem with a circular import between twisted.cred.credentials and twisted.web.iweb.  The solution used in the patch is a hack as I'm not sure what the best way to resolve the conflict is.  Would it be acceptable to move the interfaces in twisted.cred.credentials into a separate interface file?

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Type     : enhancement
Component: core
Keywords : 
Priority : low
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4060