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/test_classes.py

@@ -1647,7 +1647,7 @@ Output:
             # as the actual value of the attribute in LDAP.
             for attr, value in attrs.items():
                 if isinstance(value, str) and value.startswith("file:"):
-                    with open(value[5:], "rb") as f:
+                    with open(value.removeprefix("file:"), "rb") as f:
                         attrs[attr] = [f.read()]
 
         ldap_patcher = mock.patch("django_auth_ldap.config.ldap.initialize")