This includes `Client.pin_message`, `Client.unpin_message` and
`Client.pins_from`. This also adds the `Message.pinned` attribute
to the `Message` object.
Logging in with an invalid token would throw a TypeError due to improper
passing of arguments to HTTPClient._token. Fix by properly passing the
keyword only bot argument.
This should have a more uniform approach to rate limit handling. Instead
of queueing every request, wait until we receive a 429 and then block
the requesting bucket until we're done being rate limited. This should
reduce the number of 429s done by the API significantly (about 66% avg).
This also consistently checks for 502 retries across all requests.
