user-provided matplotlib backend names list causes TypeError: unhashable type: 'list'

[timpol]

Description of the issue

• Output of pyinstaller --version: 5.4.1
• Version of Python: 3.7.9
• Platform: Windows 10

When implementing the matplotliib backend hook configuration example in the docs:

a = Analysis(
    ["my-matplotlib-app.py"],
    ...,
    hooksconfig={
        "matplotlib": {
            #"backends": "auto",  # auto-detect; the default behavior
            # "backends": "all",  # collect all backends
            # "backends": "TkAgg",  # collect a specific backend
            "backends": ["TkAgg", "Qt5Agg"],  # collect multiple backends
        },
    },
    ...,
)

I get the following exception:

  File "C:\Users\...\lib\site-packages\PyInstaller\hooks\hook-matplotlib.backends.py", line 192, in hook
    logger.info("Matplotlib backend selection method: %s", _method_names.get(backends_method, 'user-provided name(s)'))
TypeError: unhashable type: 'list'

I think perhaps this is just an issue with the logger.info call?

I'm very inexperienced with GitHub sorry, otherwise I would try to submit a PR.