Support Python 3.12

[cneud]

With aot distutils being removed from Python 3.12 (see PEP 632), most dependencies require updates.

• ocrd: not supporting Python 3.12 yet
• numpy: the last version supporting Python 3.8 is 1.24.4 and the first version supporting Python 3.12 is 1.26.0.
• scikit-learn: only version 1.3.2 supports Python 3.8-3.12.
• tensorflow: the last version supporting Python 3.8 is 2.13.1 and the first version supporting Python 3.12 is 2.16.1 (which requires updating Keras from v2 to v3...).
• matplotlib: the last version supporting Python 3.8-3.12 is 3.7.5, releases >=3.8.0 no longer support Python 3.8.

Still need to check imutils.

Python 3.8 eol is 2024-10.

[cneud]

• tensorflow: the last version supporting Python 3.8 is 2.13.1 and the first version supporting Python 3.12 is 2.16.1 (which requires updating Keras from v2 to v3...)

https://keras.io/guides/migrating_to_keras_3/#going-from-keras-2-to-keras-3-with-the-tensorflow-backend