[3.14] gh-146452: Improve locking granularity in pickle's batch_dict_… (#150062)

[3.14] gh-146452: Improve locking granularity in pickle's batch_dict_… (#150062)

[3.14] gh-146452: Improve locking granularity in pickle's batch_dict_exact and fix race condition (GH-150025)

Remove assertion that could fail in rare race condition.

Replace the coarse critical section wrapping the entire function with
fine-grained sections covering only PyDict_Next + Py_INCREF.
Also handle PyDict_Next returning 0 in the single-item fast path.
(cherry picked from commit 57a0e570d36f41b953a91bbaf4262a5d05d0391b)