diff --git a/payment_sixpay/indico_sixpay/__init__.py b/payment_sixpay/indico_sixpay/__init__.py
index 592baf9..64a76f2 100644
--- a/payment_sixpay/indico_sixpay/__init__.py
+++ b/payment_sixpay/indico_sixpay/__init__.py
@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 #
 # This file is part of the SixPay Indico EPayment Plugin.
 # Copyright (C) 2017 - 2018 Max Fischer
diff --git a/payment_sixpay/indico_sixpay/blueprint.py b/payment_sixpay/indico_sixpay/blueprint.py
index 1bbae7e..b948f5d 100644
--- a/payment_sixpay/indico_sixpay/blueprint.py
+++ b/payment_sixpay/indico_sixpay/blueprint.py
@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 #
 # This file is part of the SixPay Indico EPayment Plugin.
 # Copyright (C) 2017 - 2018 Max Fischer
@@ -17,7 +16,6 @@
 # along with SixPay Indico EPayment Plugin;
 # if not, see <http://www.gnu.org/licenses/>.
 """Definition of callbacks exposed by the Indico server."""
-from __future__ import unicode_literals
 
 from indico.core.plugins import IndicoPluginBlueprint
 
diff --git a/payment_sixpay/indico_sixpay/plugin.py b/payment_sixpay/indico_sixpay/plugin.py
index 15d08c4..ed14ad5 100644
--- a/payment_sixpay/indico_sixpay/plugin.py
+++ b/payment_sixpay/indico_sixpay/plugin.py
@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # This file is part of the SixPay Indico EPayment Plugin.
 # Copyright (C) 2017 - 2018 Max Fischer
 #
@@ -22,7 +21,6 @@ The entry point for indico is the :py:class:`~.SixpayPaymentPlugin`.
 It handles configuration via the settings forms, initiates payments
 and provides callbacks for finished payments via its blueprint.
 """
-from __future__ import absolute_import, unicode_literals
 
 import requests
 from urllib.parse import urljoin
@@ -64,7 +62,7 @@ from .utility import (get_request_header, get_terminal_id, gettext, provider, sa
 #   - validators: Input validation, see wtforms.validators
 #   - description: help text of the field, an internationalised text
 
-class FormatField(object):
+class FormatField:
     """Validator for format fields, i.e. strings with ``{key}`` placeholders.
 
     :param max_length: optional maximum length, checked on a test formatting
@@ -112,13 +110,13 @@ class FormatField(object):
         try:
             test_format = field.data.format(**self.field_map)
         except KeyError as err:
-            raise ValidationError('Invalid format string key: {}'.format(err))
+            raise ValidationError(f'Invalid format string key: {err}')
         except ValueError as err:
-            raise ValidationError('Malformed format string: {}'.format(err))
+            raise ValidationError(f'Malformed format string: {err}')
         if len(test_format) > self.max_length:
             raise ValidationError(
                 'Too long format string:'
-                ' shortest replacement with {0}, expected {1}'
+                ' shortest replacement with {}, expected {}'
                 .format(
                     len(test_format), self.max_length
                 )
@@ -301,7 +299,7 @@ class SixpayPaymentPlugin(PaymentPluginMixin, IndicoPlugin):
             'event_id': registration.event_id,
             'event_title': registration.event.title,
             'eventuser_id':
-                'e{0}u{1}'.format(registration.event_id, registration.user_id),
+                f'e{registration.event_id}u{registration.user_id}',
             'registration_title': registration.registration_form.title
         }
 
diff --git a/payment_sixpay/indico_sixpay/request_handlers.py b/payment_sixpay/indico_sixpay/request_handlers.py
index 37037b7..98a2786 100644
--- a/payment_sixpay/indico_sixpay/request_handlers.py
+++ b/payment_sixpay/indico_sixpay/request_handlers.py
@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 #
 # This file is part of the SixPay Indico EPayment Plugin.
 # Copyright (C) 2017 - 2018 Max Fischer
@@ -17,7 +16,6 @@
 # along with SixPay Indico EPayment Plugin;
 # if not, see <http://www.gnu.org/licenses/>.
 """Callbacks for asynchronous replies by Saferpay and to redirect the user."""
-from __future__ import unicode_literals
 
 import requests
 from urllib.parse import urljoin
@@ -84,12 +82,12 @@ class SixPayResponseHandler(BaseRequestHandler):
 
     def __init__(self):
         """Initialize request handler."""
-        super(SixPayResponseHandler, self).__init__()
+        super().__init__()
         # registration context is not initialised before `self._process_args`
         self.sixpay_url = None  # type: str
 
     def _process_args(self):
-        super(SixPayResponseHandler, self)._process_args()
+        super()._process_args()
         self.sixpay_url = get_setting('url')
 
     def _process(self):
@@ -325,7 +323,7 @@ class UserSuccessHandler(SixPayResponseHandler):
             self._process_confirmation()
         except TransactionFailure as err:
             current_plugin.logger.warning(
-                'SixPay transaction failed during %s: %s' % (
+                'SixPay transaction failed during {}: {}'.format(
                     err.step, err.details
                 )
             )
diff --git a/payment_sixpay/indico_sixpay/utility.py b/payment_sixpay/indico_sixpay/utility.py
index cf86594..613630c 100644
--- a/payment_sixpay/indico_sixpay/utility.py
+++ b/payment_sixpay/indico_sixpay/utility.py
@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 #
 # This file is part of the SixPay Indico EPayment Plugin.
 # Copyright (C) 2017 - 2018 Max Fischer
@@ -17,7 +16,6 @@
 # along with SixPay Indico EPayment Plugin;
 # if not, see <http://www.gnu.org/licenses/>.
 """Utility functions used by the Sixpay payment plugin."""
-from __future__ import division, unicode_literals
 
 import uuid