The library is not thread safe

[socketpair]

ObjectIdentifier::_name uses lazy import, which is not thread-safe.

Exactly the same issue: kjd/idna#106

I think, could be solved by using Threading.lock...or, better, reorganize code to eliminate circular imports.

Also, it would be nice to make the property public, i.e. remove underscore from beginning.

[reaperhulk]

We don't intend to make this property public as we don't want to commit to the names as public surface. That said, moving _OID_NAMES into _oid.py will resolve the lazy import and should simplify the code anyway. If you want to submit a PR for that I'd be happy to review.