tp_clear
--------

Objects don't have tp_clear; that's because there shouldn't be
a situation when there's a ref cycle between them -- the way
the data is serialized on the wire precludes that.

Furthermore, all edgedb.datatypes objects are immutable, so
it's not possible to create a ref cycle with only them in the
reference chain by using the public API.

If we ever have an entity cache the situation might change,
but for now there's no need for tp_clear.
