The regression was introduced in commit 8912c182455de83e27d5c120639ec91b18247913

$ git checkout v3.8.4
$ ./configure -C
$ make
$ $ ./python bpo41295.py 
Traceback (most recent call last):
  File "bpo41295.py", line 14, in <module>
    obj.test = True
  File "bpo41295.py", line 3, in __setattr__
    type.__setattr__(cls, key, value)
TypeError: can't apply this __setattr__ to DefaultMeta object

$ git revert 8912c182455de83e27d5c120639ec91b18247913
[detached HEAD 8d3c7847a0] Revert "bpo-39960: Allow heap types in the "Carlo Verre" hack check that override "tp_setattro()" (GH-21092) (GH-21339)"
 3 files changed, 11 insertions(+), 117 deletions(-)
$ make
$ ./python bpo41295.py 
True