FileCapture: accept slicing

[hpierre001]

It would be good if FileCapture could handle slice too instead of just index

[imaginaryBuddy]

hey, I found a workaround this

from itertools import islice
cap = pyshark.FileCapture("...")
cap_sliced = islice(cap, 0, 10)
cap_sliced_list = [*cap_sliced]

and you'll be able to index from cap_sliced_list