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. 