Is there a way to remove xml header added by ElementTree.tostring method?

Before writing processed data to xml I do some formatting so it would all look nice in the result xml document.

import xml.etree.ElementTree as et
import xml.dom.minidom as mdom
(...)

for i in range(10):
    root = et.Element("main")
    (...)

    ugly_xml = et.tostring(root, 'utf-8', method='xml')
    parsed_xml = mdom.parseString(ugly_xml)
    nice_xml = parsed_xml.toprettyxml(indent=" " * 3)
    with open('test.xml', 'a') as f:
         f.write(nice_xml)

However the result file obviously has duplicate xml headers.

<?xml version="1.0" ?>
(...)
<?xml version="1.0" ?>
(...)
<?xml version="1.0" ?>

Is there a way not to print xml header with tostring method? The docs didn't provide any info except that I can just try different types like 'html' or 'text'.