add_methods(), add_members(), and add_getset() used PyDict_SetItemString()
to register descriptor to the type's dict.
So descr_new() and PyDict_SetItemString() creates interned unicode from same
C string.

This patch takes interned unicode from descriptor, and use PyDict_SetItem()
instead of PyDict_SetItemString().

python_startup_no_site:
  default: Median +- std dev: 12.7 ms +- 0.1 ms
  patched: Median +- std dev: 12.5 ms +- 0.1 ms