Speedup creating Item objects (#165)
Speedup creating Item objects (#165)

According to vmprof (https://github.com/vmprof/vmprof-python) over
a large code base, about 20% of the time was spent in Item:__init__,
use slots to reduce the time spent in Item:__init__ to 1%
jendrikseipp/vulture,jendrikseipp/vulture