[2.1.x] Refs #27795 -- Removed force_bytes() usage in sessions.

SessionBase.decode() is the inverse operation to SessionBase.encode().
As SessionBase.encode() always returns a string, SessionBase.decode()
should always be passed a string argument. Fixed the file backend, which
was the only backend still passing a bytestring.

Backport of bdae19cf from master

django/contrib/sessions/backends/base.py

@@ -10,7 +10,6 @@ from django.utils import timezone
 from django.utils.crypto import (
     constant_time_compare, get_random_string, salted_hmac,
 )
-from django.utils.encoding import force_bytes
 from django.utils.module_loading import import_string
 
 # session_key should not be case sensitive because some backends can store it
@@ -98,7 +97,7 @@ class SessionBase:
         return base64.b64encode(hash.encode() + b":" + serialized).decode('ascii')
 
     def decode(self, session_data):
-        encoded_data = base64.b64decode(force_bytes(session_data))
+        encoded_data = base64.b64decode(session_data.encode('ascii'))
         try:
             # could produce ValueError if there is no ':'
             hash, serialized = encoded_data.split(b':', 1)

django/contrib/sessions/backends/file.py

@@ -75,7 +75,7 @@ class SessionStore(SessionBase):
     def load(self):
         session_data = {}
         try:
-            with open(self._key_to_file(), "rb") as session_file:
+            with open(self._key_to_file(), "r", encoding="ascii") as session_file:
                 file_data = session_file.read()
             # Don't fail if there is no data in the session file.
             # We may have opened the empty placeholder file.

tests/sessions_tests/tests.py

@@ -311,7 +311,7 @@ class SessionTestsMixin:
         self.assertEqual(self.session.decode(encoded), data)
 
     def test_decode_failure_logged_to_security(self):
-        bad_encode = base64.b64encode(b'flaskdj:alkdjf')
+        bad_encode = base64.b64encode(b'flaskdj:alkdjf').decode('ascii')
         with self.assertLogs('django.security.SuspiciousSession', 'WARNING') as cm:
             self.assertEqual({}, self.session.decode(bad_encode))
         # The failed decode is logged.