The ultimate solution was to uninstall homebrew, since there is no obvious way to get it to use the right version of Tcl/Tk (8.5.18). And then to install the python binary from python.org, which does pick up the right version of Tcl/Tk. 

I can't remember why I was using homebrew, but the instructions at https://www.python.org/download/mac/tcltk/ (where you get pointed from IDLE) were a little unclear. I didn't realize that "if you are using OS X 10.9 or later and a Python from a python.org 64-bit/32-bit installer" didn't refer to the Python that came with macOS, or that I had to (or even could) install the python.org binary on top of the Python that was already there.