fix: tail(0, seq) should return empty instead of the whole sequence

[kaizeenn]

Summary

Fixes #626.

tail(0, seq) returns the whole sequence for sliceable inputs and empty for iterables — inconsistent and wrong.

Root cause

tail(n, seq) is implemented as seq[-n:]. When n == 0, seq[-0:] is seq[0:], returning the entire sequence. Non-sliceable iterables use deque(seq, 0) which correctly returns empty, making the behavior inconsistent across input types.

tail(0, [10, 20, 30])        # [10, 20, 30]   (wrong)
tuple(tail(0, iter([10, 20, 30])))   # ()      (correct)

Fix

Handle n == 0 as a special case before the main logic:

• For sliceable types: return seq[:0] (empty of the same type)
• For non-sliceable iterables: return () (empty tuple, consistent with deque fallback)

Test evidence

$ python3 -m pytest toolz/tests/test_itertoolz.py -v --tb=short
...
50 passed in 0.14s

$ python3 -m pytest toolz/tests/
...
180 passed, 1 failed in 0.49s

(The only failure is test_has_version which requires the package to be installed.)