It works! I tried your suggestion of installing the five software packages:
sudo apt-get install libi2c-dev python-serial i2c-tools python-smbus python3-smbus -y
The newest version of python-serial had already been installed, but the other four packages were new. Before I ran the above command, I found those four packages under /var/cache/apt/archives/ already, so I wonder whether my original installation of the PivotPi software was interrupted before it finished.
Anyway, after those four packages were installed, I was able to pivot the servo and turn on/off the LEDs on the PivotPi board with Python commands. Great! Now on to building stuff with these servos.
Thanks for the help,