Document differences between urllib3 and requests transport backends

[edmorley]

The transport page mentioned that one can choose between the (default) urllib3 transport backend and a requests-based one:
https://elasticsearch-py.readthedocs.io/en/master/transports.html

However it's not clear as to the advantages/disadvantages of one over the other.

It would be great to add a couple of sentences explaining what their differences are, and use-cases for both :-)

[honzakral]

Well, ideally you'd always use urllib3, it's only when you need requests for it's extra features (using some plugin like requests-aws4auth for example) that you should consider requests backend. I will try to add a sentence explaining this to the docs, thanks for pointing out the omission!

[edmorley]

Ah that makes sense, thank you.

One other difference that just occurred to me - when using requests you can simply pass verify_certs=True and requests handles the setup of certifi (using its vendored copy), rather than needing to do so manually with the urllib3 backend (which #403 would ideally resolve).

[edmorley]

That's great, thank you :-)