bpo-40528: fix is_simple(sum)s behavior for attributes (GH-26918)

bpo-40528: fix is_simple(sum)s behavior for attributes (GH-26918)

This is something I noticed while (now discontinued) experimenting
with the idea of annotating operators with location information. Unfortunately
without this addition, adding any `attributes` to stuff like `unaryop`
doesn't change anything since the code assumes they are singletons and
caches all instances. This patch fixes this assumption with including
the attributes as well as constructor fields.