Fix mod_repo crash on SourceEntry.architectures

Fix failure when source entry object does not have the `architectures`
attribute; this attribute is tacked on if present in `kwargs`, but does
not exist on the SourceEntry class originally, and so mod_repo was
failing when `architectures` was not supplied.
Merge pull request #3634 from gthb/mod_repo.fix-architectures-crash

Fix mod_repo crash on SourceEntry.architectures