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/test_fixtures.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 import json
 import os
 import re
@@ -289,7 +288,7 @@ def destroy_leaked_test_databases(expiry_time: int = 60 * 60) -> int:
     databases_in_use = set()  # type: Set[str]
     for file in files:
         if round(time.time()) - os.path.getmtime(file) < expiry_time:
-            with open(file, "r") as f:
+            with open(file) as f:
                 for line in f:
                     databases_in_use.add('zulip_test_template_{}'.format(line).rstrip())
         else: