iotests: Require Python 3.6 or later

Running iotests is not required to build QEMU, so we can have stricter
version requirements for Python here and can make use of new features
and drop compatibility code earlier.

This makes qemu-iotests skip all Python tests if a Python version before
3.6 is used for the build.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

tests/qemu-iotests/check

@@ -633,6 +633,12 @@ then
     export SOCKET_SCM_HELPER="$build_iotests/socket_scm_helper"
 fi
 
+python_usable=false
+if $PYTHON -c 'import sys; sys.exit(0 if sys.version_info >= (3,6) else 1)'
+then
+    python_usable=true
+fi
+
 default_machine=$($QEMU_PROG -machine help | sed -n '/(default)/ s/ .*//p')
 default_alias_machine=$($QEMU_PROG -machine help | \
    sed -n "/(alias of $default_machine)/ { s/ .*//p; q; }")
@@ -809,7 +815,12 @@ do
         start=$(_wallclock)
 
         if [ "$(head -n 1 "$source_iotests/$seq")" == "#!/usr/bin/env python" ]; then
-            run_command="$PYTHON $seq"
+            if $python_usable; then
+                run_command="$PYTHON $seq"
+            else
+                run_command="false"
+                echo "Unsupported Python version" > $seq.notrun
+            fi
         else
             run_command="./$seq"
         fi