Thanks, schef, for trying my test score and letting me know your findings. I had spent so much time the last few days researching and working on the process of installing a virtual Linux OS (I originally chose Ubuntu 15.04 but ended up with Linux Mint Cinnamon 18.0 32-bit) and getting Python 3 to work in that OS that I completely forgot about the possibility that my main problem might be with the version of MuseScore that I’m using. However, I had planned to also install MuseScore 3 on my virtual Linux OS so that I could do all of my file converting and testing in that environment. But to my dismay, I discovered that I should have installed the 64-bit version of Mint because a 32-bit version of MuseScore 3 is not available for Linux. So, I finally decided to install MuseScore 3 64-bit for Windows 10 in a different folder than MuseScore 2.1 so that I wouldn’t have to convert all of my 2.1 scores to version 3. (This is actually recommended in an article on the MuseScore website.) After I opened the 2.1 test score and saved it as a 3.0 score, I tried your script on the 3.0 score. Sure enough, it worked without any issues whatsoever.
If I had done this last week, I could have saved myself countless hours of work along with lots of stress and grief. Nevertheless, all of that work was not in vain because I now have a fully-functional virtual LInux OS installed on my PC that I can use for other things. Plus, I learned a lot in the process and benefited greatly from the mental challenge. So, thanks for your amazing conversion script, and thanks for figuring out why it wasn’t working for me.