The Steamworks source files are required for building the SteamworksPy libraries and those are only available to Steamworks account holders.
A Steamworks account is free and can be registered at https://partner.steamgames.com
The source files can be downloaded here (log-in required): https://partner.steamgames.com/downloads/steamworks_sdk_147.zip (v1.47)
Unpack the archive and place the contents of:
- /sdk/public/steam in SteamworksPy/library/sdk/steam
- /sdk/redistributable_bin/%your_os% in SteamworksPy/library/sdk/redist