b2 backend uses deprecated "b2" package

B2 moved the API from "b2" package into a separate "b2sdk" package. Using the old "b2" package is now deprecated. [1]

b2backend.py currently depends on both "b2" and "b2sdk", but use of "b2" is enforced and "b2sdk" isn't used at all.

The attached patch uses "b2sdk" as the primary dependency. If the new "b2sdk" module isn't available, it falls back to using the old "b2" in order to keep backward compatibility with older installations.

[1] - https://github.com/Backblaze/B2_Command_Line_Tool/blob/master/b2/_sdk_deprecation.py