use force_str on xml fragment (#44931)

src/authentic2_gnm/templatetags/gnm.py

@@ -15,9 +15,9 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import xml.etree.ElementTree as ET
-
 from django import template
 from django.conf import settings
+from django.utils.encoding import force_str
 from django.utils.six.moves.urllib import parse as urlparse
 
 register = template.Library()
@@ -30,7 +30,7 @@ def is_coming_for(request):
         next_url = request.GET.get('next')
     else:
         try:
-            authn_request = ET.fromstring(request.session['saml:authnRequest'])
+            authn_request = ET.fromstring(force_str(request.session['saml:authnRequest']))
             next_url = authn_request.findall(
                     '{urn:oasis:names:tc:SAML:2.0:protocol}Extensions/{https://www.entrouvert.com/}next_url')[0].text
         except (KeyError, IndexError):