How to properly format xml file using lxml?

Question

I have two xml templates: annotation.xml and object.xml.

<annotation>
    <filename>test.jpg</filename>
    <source>
        <annotation>test.py</annotation>
    </source>
    <size>
        <width>256</width>
        <height>256</height>
        <depth>3</depth>
    </size>
</annotation>

<object>
    <name>1</name>
    <bndbox>
        <xmin>0</xmin>
        <ymin>0</ymin>
        <xmax>1</xmax>
        <ymax>1</ymax>
    </bndbox>
</object>

I am reading these two templates using lxml and building an output xml file that includes one or several objects inside the annotation. Here is a minimal working example:

from lxml import etree

# parse xml templates
annotation = etree.parse('annotation.xml').getroot()
obj = etree.parse('object.xml').getroot()

# append object to annotation
annotation.append(obj)

# save output file
tree = etree.ElementTree(annotation)
tree.write('test.xml', encoding='utf-8', xml_declaration=True)

Although this works, the result I am getting does not have a proper format and looks something like this:

<?xml version='1.0' encoding='UTF-8'?>
<annotation>
    <filename>test.jpg</filename>
    <source>
        <annotation>test.py</annotation>
    </source>
    <size>
        <width>256</width>
        <height>256</height>
        <depth>3</depth>
    </size>
<object>
    <name>1</name>
    <bndbox>
        <xmin>0</xmin>
        <ymin>0</ymin>
        <xmax>1</xmax>
        <ymax>1</ymax>
    </bndbox>
</object></annotation>

whereas I would expect the following formatted file:

<?xml version='1.0' encoding='UTF-8'?>
<annotation>
    <filename>test.jpg</filename>
    <source>
        <annotation>test.py</annotation>
    </source>
    <size>
        <width>256</width>
        <height>256</height>
        <depth>3</depth>
    </size>
    <object>
        <name>1</name>
        <bndbox>
            <xmin>0</xmin>
            <ymin>0</ymin>
            <xmax>1</xmax>
            <ymax>1</ymax>
        </bndbox>
    </object>
</annotation>

I've also tried using etree.tostring(tree, pretty_print=True) and then saving the file but had no luck and the file looks the same.