Currently we're using Bugzilla's XMLRPC API through python-bugzilla. This API is deprecated and will be removed. Instead, the REST API should be used. python-bugzilla allows to use it, but our current queries seems incompatible with it and will require some changes (or fixes in python-bugzilla). Oraculum already uses this approach (REST API through python-bugzilla).

Figure out what needs to be changed and make the switch.

This change might also resolve performance issues mentioned in #183, i.e. allow us to do "quick" bugzilla syncs again (if we find it meaningful).