Right way to write string into UTF-8 file?

[snippsat]

You don't have encoding='utf-8' when you read the file as i show.
Test with your file.

with open('input.gpx') as f:
    print(f.read())

Output:
<?xml version="1.0" encoding="UTF-8"?>
<desc>Départ entre 7 et 8h</desc>
</gpx>

Fix:

with open('input.gpx', encoding='utf-8') as f:
    print(f.read())

Output:
<?xml version="1.0" encoding="UTF-8"?>
<desc>Départ entre 7 et 8h</desc>
</gpx>

As it's a .xml file,BS parser test.

from bs4 import BeautifulSoup

soup = BeautifulSoup(open('input.gpx', encoding='utf-8'), 'xml')
print(soup.find('desc').text)

Output:
Départ entre 7 et 8h