WooHoo - My MicroPython board showed up today. Here it is, enclosed in a SliceCase.
A dab of hot-glue holds the switch in place on top of the processor.
Up until now, I've been using a NetDuino Plus 2 which has the same processor as the pyboard.
Now, when I want to flash a new version of MicroPython (which I do quite frequently) I can press BOOT0 and RESET, and the board comes up in the DFU bootloader.