Depend on system pugixml. Upstream bundled their own version of pugixml. On Debian, we have a maintained packages of this library, so use that instead.
Install to correct site-packages dir, based on Python version. The Debian hack in CMakeLists was removed upstream, but python3 doesn't report a Python-specific site-packages folder. This is required, because SIP packages need to be compiled and installed for every Python version separately.