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

首先比较 frozendict 的 getnewargs 与 frozenset 的 reduce 的行为，以及协议 0 和 1 的 pickle.dumps 入口点。为两种协议添加回归覆盖，并确认 frozendict 可以被序列化和恢复，同时检查现有的关联 PR gh-155325，了解已经在进行的工作。