Remove some python2 --> python3 leftovers

Remove some __future__ imports and the six compability layer (which was
not mentioned in the requirements.txt anyway).

clear_contest.py

@@ -1,6 +1,4 @@
 #!/usr/bin/env python
-from __future__ import print_function
-
 # prepare environment
 import sys
 sys.path.append("..")

contest/apps.py

@@ -1,5 +1,3 @@
-from __future__ import unicode_literals
-
 from django.apps import AppConfig
 
 

contest/models.py

@@ -1,5 +1,3 @@
-from __future__ import unicode_literals
-
 import datetime
 
 from django.contrib.auth.models import AbstractUser

contest/validators.py

@@ -1,7 +1,6 @@
 from django.core import validators
-from django.utils import six
 from django.utils.deconstruct import deconstructible
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 import re
 
@@ -13,7 +12,8 @@ class CallUsernameValidator(validators.RegexValidator):
         'Enter a valid Callsign as Username, ALL UPPERCASE, if needed with -1 / -2,'
         'e.g. DL7BST, DN1BER-1, DL/OE1FOO.'
     )
-    flags = re.ASCII if six.PY3 else 0
+    flags = re.ASCII
+
 
 @deconstructible
 class CallLogValidator(validators.RegexValidator):
@@ -22,4 +22,4 @@ class CallLogValidator(validators.RegexValidator):
         'Enter a valid callsign, ALL UPPERCASE, if needed with -1 / -2,'
         'e.g. DL7BST, DN1BER-1, DL/OE1FOO, DN1FTW-1/p'
     )
-    flags = re.ASCII if six.PY3 else 0
+    flags = re.ASCII