readme : update Python version to 3.11 for Core ML support [no -ci]

[danbev]

This commit updates the recommended version of Python to 3.11 for Core ML conversion support. It also adds the -e flag to the generate-coreml-model.sh script to ensure that the script exits on the first error.

The motivation for this that when following the installation instructions using Python 3.10 I get the following error:

(venv) $ ./models/generate-coreml-model.sh base.en

A module that was compiled using NumPy 1.x cannot be run in
NumPy 2.1.3 as it may crash. To support both 1.x and 2.x
versions of NumPy, modules must be compiled with NumPy 2.0.
Some module may need to rebuild instead e.g. with 'pybind11>=2.12'.

If you are a user of the module, the easiest solution will be to
downgrade to 'numpy<2' or try to upgrade the affected module.
We expect that some modules will need time to support NumPy 2.

Traceback (most recent call last):  File "/whisper-work/models/convert-whisper-to-coreml.py", line 2, in <module>
    import torch
  File "/whisper-work/venv/lib/python3.10/site-packages/torch/__init__.py", line 870, in <module>
    from . import _masked
  File "/whisper-work/venv/lib/python3.10/site-packages/torch/_masked/__init__.py", line 420, in <module>
    def sum(input: Tensor,
  File "/whisper-work/venv/lib/python3.10/site-packages/torch/_masked/__init__.py", line 223, in _apply_docstring_templates
    example_input = torch.tensor([[-3, -2, -1], [0, 1, 2]])
/whisper-work/venv/lib/python3.10/site-packages/torch/_masked/__init__.py:223: UserWarning: Failed to initialize NumPy: _ARRAY_API not found (Triggered internally at  /Users/distiller/project/pytorch/torch/csrc/utils/tensor_numpy.cpp:68.)
  example_input = torch.tensor([[-3, -2, -1], [0, 1, 2]])
Minimum required torch version for importing coremltools.optimize.torch is 2.1.0. Got torch version 1.11.0.
Traceback (most recent call last):
  File "/whisper-work/models/convert-whisper-to-coreml.py", line 4, in <module>
    import coremltools as ct
  File "/whisper-work/venv/lib/python3.10/site-packages/coremltools/__init__.py", line 120, in <module>
    from . import converters, models, optimize, proto
  File "/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/__init__.py", line 7, in <module>
    from . import libsvm, sklearn, xgboost
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/xgboost/__init__.py", line 6, in <module>
    from ._tree import convert
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/xgboost/_tree.py", line 9, in <module>
    from ._tree_ensemble import convert_tree_ensemble as _convert_tree_ensemble
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/xgboost/_tree_ensemble.py", line 11, in <module>
    from ...models.tree_ensemble import TreeEnsembleClassifier
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/models/__init__.py", line 6, in <module>
    from . import (
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/models/ml_program/__init__.py", line 6, in <module>
    from . import compression_utils
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/models/ml_program/compression_utils.py", line 8, in <module>
    from coremltools.converters.mil.mil import Operation as _Operation
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/__init__.py", line 7, in <module>
    from .frontend.tensorflow.tf_op_registry import register_tf_op
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/frontend/__init__.py", line 6, in <module>
    from . import tensorflow, tensorflow2, torch
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/frontend/torch/__init__.py", line 11, in <module>
    from . import ops, quantization_ops
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/frontend/torch/ops.py", line 36, in <module>
    from .internal_graph import InternalTorchIRGraph, InternalTorchIRNode
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/frontend/torch/internal_graph.py", line 15, in <module>
    from .exir_utils import extract_io_from_exir_program
  File "/Users/danbev/work/ai/whisper-work/venv/lib/python3.10/site-packages/coremltools/converters/mil/frontend/torch/exir_utils.py", line 99, in <module>
    ) -> Dict[str, torch.fx.Node]:
AttributeError: module 'torch' has no attribute 'fx'

Using Python3.11 the conversion script runs without any errors.