FIX: Replace deprecated np.object with builtin object

[AMAN194701]

Problem

np.object was removed in NumPy 1.24. This causes an AttributeError
on any modern NumPy installation, making mpl-playback completely
broken out of the box.

Fix

Replace np.object with the builtin object in two places:

• mpl_playback/util.py: listify_dict()
• mpl_playback/record.py: line 59

Tested on

• NumPy 1.26.4
• Python 3.12.6
• Matplotlib 3.10.7

[AMAN194701]

Hi maintainers,
Both CI failures appear to be pre-existing issues unrelated to this PR:
Read the Docs fails due to an invalid readthedocs.yml configuration (build.os missing)
pre-commit.ci fails due to an invalid Poetry config in pyproject.toml

Both issues reproduce on the current main branch without my changes.
Happy to fix these in a separate PR if that would be helpful.