dh_sip3 for python3

[Scott Kitterman]

We've had dh_sip now for a couple of years.  Last year python3 packages were 
added for sip4, but without the companion dh_sip3.  I've now put it together 
and it works ~like dh_sip.  

For python3 packages that use sip, add the ${sip3:Depends} substitution 
variable and at build time this will be substituted for the SIP API version 
the package was built against (currently it would produce sip-py3api-8.1.  
Don't forget to add a call to it in debian/rules.

Scott K