functions. Changed some ">= 2.1" stix version semantics to be
"== 2.1", because we don't have any version >= 2.1, so they are
currently equivalent, and the is_*() functions don't support
STIX version ranges. They only support exact versions. We can
look at this again if a newer STIX version ever emerges.
Also added a class_for_type() function to the registry module,
which was useful for the versioning module changes described
above. I thought that function would be helpful in the parsing
module, to simplify code there, so I changed that module a bit
to use it.
473e7d0068