deps: Downgrade python3-saml

Recent python3-saml versions block lxml upgrade, which in turn contains
security fixes for  GHSA-wrxv-2j5q-m38w.
The version restriction seems to be caused by binary wheel not
compatible with some distros (see
SAML-Toolkits/python3-saml#292). As we build lxml
from the source, we're not affected by this.

Once python3-saml raises the restriction (for example by
SAML-Toolkits/python3-saml#309), we can switch back
to the latest version.

requirements.txt

@@ -36,7 +36,7 @@ psycopg2==2.9.3
 Pygments==2.12.0
 python-akismet==0.4.3
 python-dateutil==2.8.2
-python3-saml==1.14.0
+python3-saml==1.12.0
 pyuca==1.2
 raven
 redis==4.3.4