keyring error from no arguments doesn't mention new diagnose operation

[micahellison]

Describe the bug
The new diagnose command in #633 doesn't appear when you run keyring by itself.

To Reproduce
Run keyring with no arguments.

Expected behavior
The error message should look something like:

keyring: error: Specify operation 'get', 'del', 'set', or 'diagnose'

Actual behavior

usage: keyring [-h] [-p KEYRING_PATH] [-b KEYRING_BACKEND] [--list-backends] [--disable] [--print-completion {bash,zsh,tcsh}]
               [{get,set,del,diagnose}] [service] [username]
keyring: error: Specify operation 'get', 'del', or 'set'.

Environment

• OS: Windows 10 Pro

$ pip list | grep keyring

keyring              24.0.0

$ keyring --list-backends
keyring.backends.fail.Keyring (priority: 0)
keyring.backends.chainer.ChainerBackend (priority: -1)
keyring.backends.Windows.WinVaultKeyring (priority: 5)

Additional context
Thanks for all your hard work on this project!