fixed requirements.txt for python3.9

requirements.txt

@@ -10,7 +10,8 @@ Flask-OAuthlib==0.9.5
 # not very good at resolving them, so we pin the versions from Debian Buster
 # for all dependencies.
 certifi==2018.8.24
-cffi==1.12.2
+#cffi==1.12.2
+cffi # v1.12.2 no longer works with python3.9. Newer versions seem to work fine.
 chardet==3.0.4
 click==7.0
 cryptography==2.6.1