With Python 3.9, the crypt function raises an OSError

See https://bugs.python.org/issue39289

Signed-off-by: Yohann D'ANELLO <ynerant@¢rans.org>
Signed-off-by: Yohann D'ANELLO <ynerant@crans.org>

cas_server/utils.py

@@ -409,7 +409,10 @@ def crypt_salt_is_valid(salt):
                 if '$' not in salt[1:]:
                     return False
                 else:
+                    try:
                         hashed = crypt.crypt("", salt)
+                    except OSError:
+                        return False
                     if not hashed or '$' not in hashed[1:]:
                         return False
                     else: