tests/qemu-iotests/check: Allow use of python3 interpreter

As we want to enforce a unique and explicit Python 3 interpreter,
we need let this script handle 'python3' too.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-3-philmd@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/qemu-iotests/check

@@ -846,7 +846,8 @@ do
 
         start=$(_wallclock)
 
-        if [ "$(head -n 1 "$source_iotests/$seq")" == "#!/usr/bin/env python" ]; then
+        if [ "$(head -n 1 "$source_iotests/$seq" | sed 's/3$//')" \
+            == "#!/usr/bin/env python" ]; then
             if $python_usable; then
                 run_command="$PYTHON $seq"
             else