bpo-37760: Convert from length-18 lists to a dataclass, in makeunicodedata. (GH-15265)

Now the fields have names!  Much easier to keep straight as a
reader than the elements of an 18-tuple.

Runs about 10-15% slower: from 10.8s to 12.3s, on my laptop.
Fortunately that's perfectly fine for this maintenance script.