Documenting Mac python 3.9 install issues

[robmarkcole]

1. ERROR: No matching distribution found for pyyaml==5.4.1. Solution pyyaml>=5.4.1

2. ERROR: No matching distribution found for lightgbm==3.3.3. Tried lightgbm>=3.3.3. Get Failed to build lightgbm ERROR: Could not build wheels for lightgbm which use PEP 517 and cannot be installed directly. For now I will comment it out. However I then get ModuleNotFoundError: No module named 'lightgbm'

Trying brew install lightgbm - successfully completes and I can then pip install lightgbm

3. ModuleNotFoundError: No module named 'sklearn'. Solution pip install scikit-learn -> to add to requirements

[robmarkcole]

I can then

 > iris demo
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