sys.platform comes from Py_GetPlatform() which comes from PLATFORM define. On Linux, this define comes from Makefile: MACHDEP variable which comes from configure. Finally, MACHDEP is defined by:

	ac_sys_system=`uname -s`
	if test "$ac_sys_system" = "AIX" \
	-o "$ac_sys_system" = "UnixWare" -o "$ac_sys_system" = "OpenUNIX"; then
		ac_sys_release=`uname -v`
	else
		ac_sys_release=`uname -r`
	fi
	ac_md_system=`echo $ac_sys_system |
			   tr -d '/ ' | tr '[A-Z]' '[a-z]'`
	ac_md_release=`echo $ac_sys_release |
			   tr -d '/ ' | sed 's/^[A-Z]\.//' | sed 's/\..*//'`
	MACHDEP="$ac_md_system$ac_md_release"