Improve usability when Packet objects are retained past the callback
Add Packet.retain() to save the packet contents in such cases
Eliminate warnings during build on py3
Add CI and basic test suite
Raise a warning, not an error, if we don't get the bufsize we want
Don't allow bind() more than once on the same NetfilterQueue, since that would leak the old queue handle
** This will be the last version with support for Python 2.7. **