tests/qemu-iotests/check: Only check for Python 3 interpreter

All the iotests Python scripts have been converted to search for
the Python 3 interpreter. Update the ./check script accordingly.

Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-13-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/qemu-iotests/check

@@ -846,8 +846,7 @@ do
 
         start=$(_wallclock)
 
-        if [ "$(head -n 1 "$source_iotests/$seq" | sed 's/3$//')" \
-            == "#!/usr/bin/env python" ]; then
+        if [ "$(head -n 1 "$source_iotests/$seq")" == "#!/usr/bin/env python3" ]; then
             if $python_usable; then
                 run_command="$PYTHON $seq"
             else