I just quickly went any checked the program I was trying to debug. I was running 'python3 package --arguments', where 'package' is the name of a directory which contains a package I was working on. 'python -m pdb package --arguments' just complains that 'package' is a directory.

Adding a 'breakpoint()' at the start of the program does get me into the debugger. I'll remember that for future (but, it's not easy to find by googling if you don't already know what you are looking for!)