Mautrix

* Added wrapper for [MSC2716]'s `/batch_send` endpoint in `IntentAPI`.
* Added some Matrix request metrics (thanks to [jaller94] in [68]).
* Added utility method for adding variation selector 16 to emoji strings the
same way as Element does (using emojibase data).

[MSC2716]: https://github.com/matrix-org/matrix-spec-proposals/pull/2716
[jaller94]: https://github.com/jaller94
[68]: https://github.com/mautrix/python/pull/68

* *(util.formatter)* Added support for parsing Matrix HTML colors.

* Added autogenerated docs with Sphinx.
* Rendered version available at https://docs.mau.fi/python/latest/
(also version-specific docs at https://docs.mau.fi/python/v0.12.3/).
* Added asyncpg to client state store unit tests.
* Fixed client state store `get_members` being broken on asyncpg (broken in 0.12.2).
* Fixed `get_members_filtered` not taking the `memberships` parameter into
account in the memory store.

* Added more control over which membership states to return in client state store.
* Added some basic tests for the client state store.
* Fixed `OlmMachine.account` property not being defined before calling `load`.

* Added default (empty) value for `unsigned` in the event classes.
* Updated the `PgStateStore` in the client module to fully implement the crypto
`StateStore` abstract class.
* The crypto module now has a `PgCryptoStateStore` that combines the client
`PgStateStore` with the abstract crypto state store.

* **Breaking change (client):** The `whoami` method now returns a dataclass
with `user_id` and `device_id` fields, instead of just returning the
`user_id` as a string.
* Added `delete` method for crypto stores (useful when changing the device ID).
* Added `DECRYPTED` step for message send checkpoints.
* Added proper user agent to bridge state and message send checkpoint requests.