[3.14] gh-144295: Fix data race in dict method lookup and global load (gh-144312) (#144346)

In `_Py_dict_lookup_threadsafe_stackref`, call `ensure_shared_on_read()` to
prevent a race between the lookup and concurrent dict resizes, which may free
the PyDictKeysObject (i.e., it ensures that the resize uses QSBR).

(cherry picked from commit e666a01ef42939f77f4c22ca47a610df5ef8b7ab)