frozendict cannot be pickled with protocols 0 and 1

Bug report

frozendict cannot be pickled with protocols 0 and 1, while frozenset can:

>>> import pickle
>>> pickle.dumps(frozendict(a=1), 0)
Traceback (most recent call last):
  ...
TypeError: cannot pickle 'frozendict' object
>>> pickle.dumps(frozenset({1}), 0)  # works

Protocols 2 through 5 already work. frozendict only defines __getnewargs__,
a protocol-2-and-later mechanism, while frozenset defines __reduce__, which
works at every protocol.

CPython versions tested on:

3.16 (main)

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-155325

首先比較 frozendict 的 getnewargs 與 frozenset 的 reduce 行為，以及協定 0 和 1 的 pickle.dumps 進入點。為兩種協定新增回歸測試涵蓋，並確認 frozendict 可以被序列化和還原，同時檢查現有的相關 PR gh-155325，了解已在進行的工作。