I have tried running the script with:

- command line (python <file>): Works without (breaks with) `import _tkinter`.
- basic python console (python): Same as with command line.
- ipython: Fails with or without `import _tkinter`. However, if for example I make the matrix in the scripts smaller or use only functions that don't resort to multithreading, code runs successfully in parallel in ipython.
- IDLE: Perhaps unsurprisingly, the code fails with or without `import _tkinter` (never terminates, or prompts endless "Your program is still running! Do you want to kill it?")

Suspecting a problem with Tcl/Tk (Apple's original 8.5.9), I rebuilt python pointing to tcl-tk installed with brew (8.6.8), which I checked with IDLE and _tkinter.TCL_VERSION() / _tkinter.TK_VERSION(). However, this did not fix the problem.