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/bugdown/include.py

@@ -1,4 +1,3 @@
-from __future__ import print_function
 import re
 import os
 from typing import Any, Dict, List
@@ -42,7 +41,7 @@ class IncludeCustomPreprocessor(IncludePreprocessor):
                             os.path.join(self.base_path, filename)
                         )
                     try:
-                        with open(filename, 'r', encoding=self.encoding) as r:
+                        with open(filename, encoding=self.encoding) as r:
                             text = r.readlines()
                     except Exception as e:
                         print('Warning: could not find file {}. Error: {}'.format(filename, e))