Postgres-tq

* The `reschedule()` method has an optional parameter to decrease the TTL if set to True (False by default).
* The `add()` and `add_many()` methods set the `can_start_at` by default to the current time of the database clock, not Python, for consistency.

* The `complete()` method now returns the count of updated tasks, 0 if it was already completed

* Restrict the type yielded by the generator to never be None

* Allow delayed schedule of tasks, use clearer name for database columns
* Add `get_many` helper to retrieve multiple tasks with a single DB call
* Add `add_many` helper to insert multiple tasks with a single transaction
* Create an index on the table, can now scale to millions of tasks
* Every method returning tasks returns the queue name too, in case multi-queue is added later
* Type hints are made available to users thanks to `py.typed`

* Adding a task now returns its UUID, previously nothing was returned

* Upgraded dependencies, now this library requires Python 3.9 (previously was 3.8)