python: Simplify with str.removeprefix, str.removesuffix.

These are available in Python ≥ 3.9.
https://docs.python.org/3/library/stdtypes.html#str.removeprefix

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/openapi/javascript_examples.py

@@ -16,7 +16,7 @@ from zerver.openapi.openapi import validate_against_openapi_schema
 def test_js_bindings(client: Client) -> None:
     os.environ["ZULIP_USERNAME"] = client.email
     os.environ["ZULIP_API_KEY"] = client.api_key
-    os.environ["ZULIP_REALM"] = client.base_url[:-5]
+    os.environ["ZULIP_REALM"] = client.base_url.removesuffix("/api/")
 
     output = subprocess.check_output(
         args=["node", "--unhandled-rejections=strict", "zerver/openapi/javascript_examples.js"],