Sprockets-postgres

This release adds metadata to setup.cfg informing tooling that there is a minimum version of setuptools that must be used to install the library.

This release attempts to fix a race condition that would occur if the database connections are remotely closed and the pool is in the process of being recreated.

Previously the behavior would result in an uncaught exception. This update should see 503 responses sent to client requests, and should prevent the uncaught exception.

- Adds a new 409 response using json-problem-details for FK constraint errors
- FIxes a bug for when you try and poll a service before Application.on_start callbacks are called

The previous release missed areas where the library was still directly depending upon `os.enivorn`.

- Updates the package to allow for the configuration variables to be passed in via Application.settings
- Adds Python 3.9 to the support matrix
- Moves from Alpha to Beta

This version transparently uses the tornado-problem-details package in the `RequestHandlerMixin`, if it's installed instead, raising `problemdetails.Problem`, instead of `tornado.web.HTTPError`.

`RequestHandlerMixin._on_postgres_error` was changed to `RequestHandlerMixin.on_postgres_error`
`RequestHandlerMixin._on_postgres_timing` was changed to `RequestHandlerMixin.on_postgres_timing`

In addition it removes the requirement for an `on_error` callback, allowing for request handlers that do not use the `RequestHandlerMixin` to catch their own exceptions and do all that work without having to override `RequestHandlerMixin.on_postgres_error`.