[davidism]

parse_list_header is more accurate in how it parses quoted and empty items according to RFC 9110 https://httpwg.org/specs/rfc9110.html#abnf.extension. Previously it just wrapped urllib.request.parse_http_list, which was a bit too simple.

• Empty parts are now discarded.
• Backslash escapes are not unconditionally removed from quoted strings. unquote_header_value is used consistently for this purpose.
• An unclosed quoted string is considered invalid and results in an empty list, rather than consuming the rest of the value. I considered raising a ValueError, but in general our parsing functions return empty values for invalid input.