Prepare release v0.7.0 (#37)

Prepare v0.7.0, containing largely an implementation for fast path
unique job insertion as implemented in #36.

Prepare release 0.6.3 (#35)

Prepare release 0.6.3 which contains various fixes for Python syntax in
README examples [1].

[1] #34

Prepare release 0.6.2

Prepare release 0.6.2, which changes the typing on `UniqueOpts.by_state`
from `list[str]` to `list[JobState]` (the enum).

Prepare release 0.6.1

Prepare release 0.6.1, including a fix for unmarshaling `AttemptError`.

Prepare release 0.6.0

Prepare release 0.6.0, which includes doc strings for most of the public
API and a new `AttemptError` data class to represent a job error.

Prepare release 0.5.0

Prepare release 0.5.0, including a new `JobState` enum and fully typed
`Job` row.

Prepare release 0.4.0

Prepare release 0.4.0, containing new tag validations [1].

[1] #23

Prepare version 0.3.0

Tee up version 0.3.0, whose largest feature is a full implementation of
`insert_many()` and `insert_many_tx()` [1].

[1] #22

Prepare version 0.2.0

Prepare release 0.2.0, which largely just includes a rename of `Args` to
`JobArgs`, a new `JobArgsWithInsertOpts` protocol for slightly nicer
typing, and a few more tests [1].

[1] #20

Cut release 0.2.0

Cut release 0.2.0, where we add usage instructions README, add job state
constants, and change return value of `insert_many()` and
`insert_many_tx()` to an integer instead of a list of jobs [1].

[1] riverqueue/river#19