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>

tools/linter_lib/pyflakes.py

@@ -1,6 +1,3 @@
-from __future__ import print_function
-from __future__ import absolute_import
-
 import argparse
 
 from typing import List