CLN: Start definition of Backend API, and implement version

[datapythonista]

xref #2678

First step towards defining the Backend API properly with abstract methods (Backend will replace Client).

To make PRs small, I define a first set of methods that Backend/Client must have, and so far I implement version.

To be able to do this step by step, I move the implementation from Client to Backend, and I created the property in the base Client, calling the Backend implementation. Eventually, connect will return Backend instead of Client, and Client will be removed.

I'm making a backward-incompatible change here, returning version as a string, instead of a setuptools object. I think this will match user expactations, since libraries rarely parse it (i.e. any_library.__version__) and it will also make our life easier with type annotations (setuptools have different version objects that can be returned).

[datapythonista]

@jreback if you don't mind prioritizing this one, this is blocking me from finishing the stuff for the release

[jreback]

looks generally ok

[datapythonista]

@jreback whenever you've got time, would be great if you can check my comments. I think this should be ready, and it's blocking me from continuing with clarifying the API for the backends. Thanks!

[jreback]

thanks @datapythonista prob should add a release note about the version string change next time.

[datapythonista]

thanks @datapythonista prob should add a release note about the version string change next time.

Good point, added it in #2895, thanks for merging anyway.