bpo-37760: Factor out the basic UCD parsing logic of makeunicodedata. (GH-15130)

bpo-37760: Factor out the basic UCD parsing logic of makeunicodedata. (GH-15130)

There were 10 copies of this, and almost as many distinct versions of
exactly how it was written.  They're all implementing the same
standard.  Pull them out to the top, so the more interesting logic
that remains becomes easier to read.