Refactor S3, replace high-level resource/session API with low-level client API

[mpenkov]

While functionally they are the same, the session/resource stuff is not safe to use across multiple threads and subprocesses, and the client stuff is. The former is a bit easier to use directly than the latter, but this does not impact the user, because smart_open is doing all the work.

I also refactored the way we pass keyword parameters to boto3. Previously, we had a separate parameter for each boto3 function. This was a pain, because this made parameter lists longer for each new function, e.g.

• resource_kwargs
• multipart_upload_kwargs
• singlepart_upload_kwargs
• object_kwargs

This PR moves all of the above parameters into a single nested dict and introduces a wrapper that transparently injects the parameters into the required function call.

This does break backwards compatibility, so will need a major version bump when releasing.

[menshikh-iv]

Awesome work @mpenkov 🔥

Just one note

[mpenkov]

@piskvorky Can you please have a look at this? I'd like to merge and release so I can use this in one of my projects.

[mpenkov]

@piskvorky Ping

[piskvorky]

I did a cursory review, the code seems good. But what about compatibility? What does this do to existing users?

[mpenkov]

This is a breaking change. People doing stuff with kwargs will have to switch from using resource paramaters to using client ones. This will mean a major version bump.

[piskvorky]

OK. And is this documented somewhere? A migration guide?

[mpenkov]

Good point. I added a section to the migration guide. Please have a look.

[mpenkov]

Updated https://github.com/RaRe-Technologies/smart_open/blob/client/MIGRATING_FROM_OLDER_VERSIONS.rst, please have a look.