Sigh. Of course after I spend hours researching and writing up a stackoverflow question, I find the right answer hidden among other answers.

RStudio > Tools > Global Options > Python > Select (your python interpreter) > Virtual Environments > ~/.virtualenvs/r-reticulate/Scripts/python.exe > Select > Apply

Now source_python(r"(C:\Users\myname\.spyder-py3\temp.py)") works like a charm.