I managed to accidentally resolve this issue. I recently updated my operating system from an old version of macOS to the new macOS Monterrey. I was occasionally getting what seemed like an unrelated error when using the terminal (not the python REPL):
xcrun: error: invalid active developer path
(/Library/Developer/CommandLineTools), missing xcrun
at: /Library/Developer/CommandLineTools/usr/bin/xcrun
I was able to resolve that by running xcode-select --install in the terminal, as described in these two posts:
Now that I installed xcode-select, I no longer see the warnings from pymc mentioned in my original post. I think I had Xcode installed before I upgraded the OS on my laptop, but I guess the upgrade process affected that.