Allow cookie authentication to work with Python 3.5

The json module started understanding bytes in Python 3.6 (with an
heuristic on which Unicode encoding the bytes are using).

We don't ufficially support Python 3.5, but since it's an easy fix and
we know better than the heuristic the encoding (UTF-8, since
everything is ASCII, really), let's decode explicitly.

cms/server/contest/authentication.py

@@ -150,7 +150,8 @@ def log_failed_attempt(msg, *args):
                 timestamp)
 
     return (participation,
-            json.dumps([username, password, make_timestamp(timestamp)]))
+            json.dumps([username, password, make_timestamp(timestamp)])
+                .encode("utf-8"))
 
 
 class AmbiguousIPAddress(Exception):
@@ -316,7 +317,7 @@ def _authenticate_request_from_cookie(sql_session, contest, timestamp, cookie):
 
     # Parse cookie.
     try:
-        cookie = json.loads(cookie)
+        cookie = json.loads(cookie.decode("utf-8"))
         username = cookie[0]
         password = cookie[1]
         last_update = make_datetime(cookie[2])
@@ -357,4 +358,5 @@ def log_failed_attempt(msg, *args):
                 timestamp)
 
     return (participation,
-            json.dumps([username, password, make_timestamp(timestamp)]))
+            json.dumps([username, password, make_timestamp(timestamp)])
+                .encode("utf-8"))