fix: accept tuples (and any sequence) in _expand_iterable, not just lists

[koteshyelamati]

Fixes#434

Problem

tabulate() raises TypeError when sequence-valued parameters like rowalign or maxheadercolwidths are passed as tuples instead of lists:

fromtabulateimporttabulatetabulate([[1, 2], [3, 4]], rowalign=("top", "bottom"))
# TypeError: can only concatenate tuple (not "list") to tupletabulate([[1, 2], [3, 4]], rowalign=["top", "bottom"]) # works

All other sequence-valued parameters (colalign, floatfmt, maxcolwidths, etc.) accept tuples, so this is an inconsistency.

Root cause

_expand_iterable does original + [default] * n. When original is a tuple, tuple + list raises TypeError.

Fix

Coerce original to a list before concatenation:

# beforereturnoriginal+ [default] * (num_desired-len(original))
# afterreturnlist(original) + [default] * (num_desired-len(original))