bpo-41944: No longer call eval() on content received via HTTP in the UnicodeNames tests (GH-22575)

Similarly to GH-22566, those tests called eval() on content received via
HTTP in test_named_sequences_full. This likely isn't exploitable because
unicodedata.lookup(seqname) is called before self.checkletter(seqname,
None) - thus any string which isn't a valid unicode character name
wouldn't ever reach the checkletter method.

Still, it's probably better to be safe than sorry.