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/openapi/test_curl_examples.py

@@ -18,7 +18,7 @@ def test_generated_curl_examples_for_success(client: Client) -> None:
         api_url=realm.uri + "/api")])
 
     for file_name in glob.glob("templates/zerver/api/*.md"):
-        documentation_lines = open(file_name, "r").readlines()
+        documentation_lines = open(file_name).readlines()
         for line in documentation_lines:
             # A typical example from the markdown source looks like this:
             #     {generate_code_example(curl, ...}