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/parallel.py

@@ -29,7 +29,7 @@ def run_parallel(job: Callable[[JobData], int],
             except OSError as e:
                 if e.errno != errno.EBADF:
                     raise
-            sys.stdin = open("/dev/null", "r")
+            sys.stdin = open("/dev/null")
             os._exit(job(item))
 
         pids[pid] = item