`pi-topd` rework: retiring `/etc/pi-top` for non-configuration files

[m-roberts]

Changes

• Build via pybuild
  • Python source now uses smbus2, with Debian packaging patching to smbus
  • All entrypoints are now defined in setup.cfg
  • Dependencies are now inherited from Python
  • All files now use relative imports
• Moved to /run for files
• Use state file

Related PRs

• SDK (dependency)
• core package (dependency)
• firmware updater (breadcrumb, service name update)
• pt-miniscreen (service name update, dpkg trigger)
• web portal (/run device info)