Make stubtest ignore __slotnames__ (#19077)

This is a cached list of names of slots added by the `copyreg` module.

See typeddjango/django-stubs#2584 for a case of
these cropping up in `django-stubs`.

Author: ngnpope

mypy/stubtest.py

@@ -1541,6 +1541,7 @@ def verify_typealias(
         "__getinitargs__",
         "__reduce_ex__",
         "__reduce__",
+        "__slotnames__",  # Cached names of slots added by `copyreg` module.
         # ctypes weirdness
         "__ctype_be__",
         "__ctype_le__",