Thank you for your wise elucidative remark. I can't believe I forgot I was running from source.
If it is a problem running from source, then it will probably be a problem later on in the official binary distributions. A number of PyQt5.5 compatibility issues were recently squelched that way, and similarly in calibre's history.
I am doing everything the recommended way -- running from source *shouldn't* be a problem. If it is anyway, Kovid will be interested at the very least.