(sgmllib.py): Partial acceptance of patch from David Leonard

(sgmllib.py):  Partial acceptance of patch from David Leonard
	<leonard@dstc.edu.au>; allows hyphen and period in the middle
	of attribute names.  Still not allowed as first character;
	as first character these are illegal in the Reference Concrete
	Syntax, and we've not identified any use of these characters as
	the first char in an attribute name in deployment on the web.