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/lib/zcommand.py

@@ -22,7 +22,7 @@ def process_zcommands(content: str, user_profile: UserProfile) -> dict[str, Any]
 
     if not content.startswith("/"):
         raise JsonableError(_("There should be a leading slash in the zcommand."))
-    command = content[1:]
+    command = content.removeprefix("/")
 
     if command == "ping":
         return {}