Update django to django 3.1, remove redundant backport code

saml2idp/codex.py

@@ -5,8 +5,6 @@
 import zlib
 import base64
 
-from django.utils.six import binary_type
-
 
 def decode_base64_and_inflate(b64string):
     decoded_data = base64.b64decode(b64string)
@@ -21,5 +19,5 @@ def deflate_and_base64_encode(string_val):
 
 def nice64(src):
     """ Returns src base64-encoded and formatted nicely for our XML. """
-    assert isinstance(src, binary_type), 'Can only encode bytes'
+    assert isinstance(src, bytes), 'Can only encode bytes'
     return base64.b64encode(src).decode('ascii')

saml2idp/xml_signing.py

@@ -9,7 +9,6 @@
 import string
 
 import OpenSSL.crypto
-from django.utils import six
 
 from . import saml2idp_metadata as smd
 from .codex import nice64
@@ -54,9 +53,6 @@ def load_private_key(config):
 
 def sign_with_rsa(private_key, data):
     digest = "sha1"
-    # This needs to be bytes on py2, str on py3. This is very odd.
-    if six.PY2:
-        digest = digest.encode('ascii')
     data = OpenSSL.crypto.sign(private_key, data, digest)
     return base64.b64encode(data).decode('ascii')