Faster dogfooding of Python SDK in `sentry`.

[antonpirker]

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.