Errors installing PyQt5 dependency on ARM Mac using pip

[wsdewitt]

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Currently pip install gctree fails for me on an ARM Mac, giving an error like this from the PyQt5 dependency. The Conda install works though. PyQt5 installs fine via Conda, but not from pip. It could be helpful to add a note/warning in the install docs that ARM Mac users currently need to install PyQt5 somehow (i.e. Conda) before installing gctree via pip (e.g. if gctree is a dependency in the setup.py of another package).