[Python] In a response body, `eTag` should be transcribed to `etag`, not `e_tag`

Using the snake-cased version `e_tag` is in contradiction to the built-in concurrency parameter `etag` - these should be aligned.
We could, for now, hard-code this particular case - however there's probably a broader discussion needed here as to whether there's ever a case that a single-letter section should be snake-cased.