The tk module has problems doing just about anything, giving the error message:
TclError: Can't find a usable init.tcl in the following directories:
/usr/local/Cellar/tcl-tk/8.6.13_4/lib/tcl8.6 /Applications/Spyder.app/Contents/lib/tcl8.6 /Applications/Spyder.app/lib/tcl8.6 /Applications/Spyder.app/Contents/library /Applications/Spyder.app/library /Applications/Spyder.app/tcl8.6.13/library /Applications/tcl8.6.13/library
This probably means that Tcl wasn't installed properly.
I re-downloaded Tcl and put the library in one of the directories it is looking for, as per the suggestions in this question. I also made sure that I have Tkinter working on my computer - running the command
python3 -m tkinter
launches the proper window, which also tells me that my version of tkinter is indeed 8.6.13, but working from Spyder yields the mentioned error message.