Using Steamworks versions 1.5.6 (v156) or higher causes generic errors/exceptions to be thrown. Using a previous version seems to work as expected.
In my case, it generically reported missing dependencies in steamworks/__init__.py at line 91 self._cdll = CDLL(library_path) # Throw native exception in case of error, which I suspect is due to incompatible newer versions of the SteamAPI files; I used 1.6.2 (v162) initially when getting the error, but switched to 1.5.5 (v155) and it worked fine.
I used the pre-compiled SteamworksPy64.dll, and my app_id was set to 480 for the default Spacewar game, to test with.