python: Modernize legacy Python 2 syntax with pyupgrade.

Generated by `pyupgrade --py3-plus --keep-percent-format` on all our
Python code except `zthumbor` and `zulip-ec2-configure-interfaces`,
followed by manual indentation fixes.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/push_notifications.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 import base64
 import binascii
 import logging
@@ -276,7 +274,7 @@ def send_android_push_notification(devices: List[DeviceToken], data: Dict[str, A
                                       priority=priority,
                                       data=data,
                                       retries=10)
-    except IOError as e:
+    except OSError as e:
         logger.warning(str(e))
         return