Faster dogfooding of Python SDK in `sentry`.

Sometimes it takes weeks until we update `sentry` after a new version of the SDK is released. `sentry` should always use the most recent version of the SDK for best dog fooding.

Set up dependabot in `sentry` to also check the Python SDK, and make sure that the Web Backend SDK gets a notifiction when dependabot creates a PR, so we can semi automate this step.