accept both bytes and str for cnfvar readers

accept both bytes and str for cnfvar readers

Make both the CNF format and the JSON readers handle bytes and
str transparently.

(Note there is still a deplorable asymmetry in the routines such
that the JSON path will convert to *str* whereas the CNF part,
assuming single-byte encoded data, will convert to *bytes*. This
is necessary for compatibility reasons until the Intranator gets
a more recent Python interpreter.)