When a session aborts, but doesn't have retry info python immediately retries. It seems to lack any sort of backoff here which may be incorrect. node attempts to.

https://github.com/googleapis/nodejs-spanner/blob/79f7855b3f6df59afe322dd578ed65f408a2fc2b/src/transaction.ts#L836

Python implementation currently returning 'None' if nothing is found:

Go: https://github.com/googleapis/google-cloud-go/blob/ab1512ccf757de1b7f7f62883d9f975dfc152cef/spanner/internal/backoff/backoff.go#L43