Well horse pucky. I plum forgot about deletes.

I've attached an update that properly clears the cache entry for the
deleted attribute for all non-shadowing subclasses. (It was a small
change.) Undef'ing ATTRCACHE_SETATTR_INVALIDATES should work now.

Re: different from 2.6: It takes advantage of the lack of "classic"
classes. That makes some things a lot easier. Also, I was looking into
updating cache entries to see if it would be faster than invalidating
all the entries for a type, among other improvements. FWIW, updating
cache entries is a little faster on my box.

Also: This may have the same problem with test_ctypes'
test_incomplete.py as #1700288 did, though I haven't seen any ctypes
tests fail.