Littlefixes

This includes some minor fixes:

* Do not require packages that are not needed for ooniprobe

* Fix bug where some scapy classes would not get imported if pypcap or libdnet were not installed

* Remove the import of _Wrappingfactory (this should have been removed some commits ago when we switched to using txsocksx instead of a custom SOCKS client) (this also fixed #28)