Drop python 2 support

During testing, I received this backtrace as a result of running
"bugz get 373219 > file.txt" on Gentoo's bugzilla:

Traceback (most recent call last):
  File "/usr/lib/python-exec/python2.7/bugz", line 72, in <module>
    sys.exit(main())
  File "/usr/lib/python-exec/python2.7/bugz", line 47, in main
    args.func(bugz, args)
  File "/usr/lib64/python2.7/site-packages/bugz/cli.py", line 281, in get
    self.showbuginfo(bug, args.attachments, args.comments)
  File "/usr/lib64/python2.7/site-packages/bugz/cli.py", line 775, in showbuginfo
    print line
UnicodeEncodeError: 'ascii' codec can't encode character u'\u0142' in position 7: ordinal not in range(128)

while researching this, I was referred to the following upstream issues:

https://bugs.python.org/issue6832
https://bugs.python.org/issue14192

This is not going to be fixed in python 2.x, so I am dropping python 2
support.