Fixing lxml to 4.7.0, it seems 4.7.1 had conflicts when the signature inside a encrypted element is validated.

See SAML-Toolkits/python3-saml#292
Also reverted dm.xmlsec.binding to 1.3.7

Author: pitbulk

setup.py

@@ -32,8 +32,8 @@
     },
     test_suite='tests',
     install_requires=[
-        'lxml>=4.7.1',
-        'dm.xmlsec.binding==2.1',
+        'lxml<4.7.1',
+        'dm.xmlsec.binding==1.3.7',
         'isodate>=0.6.1',
         'defusedxml>=0.7.1',
     ],