Introduce the new cs3-python-client library

[glpatcern]

This PR is to introduce the https://github.com/cs3org/cs3-python-client library contributed by @rawe0.

As part of this, some refactoring and simplification has taken place:

• Simplified xattrs handling: dropped the need for an independent getxattr() API, as well as the caching recently introduced, as the xattrs are always queried next to a stat call.
• Removed versioninv flag on stat, assuming it's always version invariant. Only xroot was able to make a distinction.
• Standardized readfile that now raises an exception instead of yielding it. Consequently dropped the peek logic.
• Standardized error messages.

@micbar a heads-up about a larger-than-usual change. I will test this with the Microsoft WOPI validator prior to merge.

[glpatcern]

Marking as ready for review given that the code is pretty much complete. Once the build passes, we can review and merge this change.

[glpatcern]

Remaining tasks:

• [x] convert the code dealing with yielded exceptions from readfile(), now that the function can raise them.
• [x] rename exception messages (ENOENT goes into the cs3 library, EXCL_ERROR gets replaced from the library)
• [x] get rid of versioninv

[glpatcern]

Following https://github.com/cs3org/cs3-python-client/pull/8 we now pass the whole test suite:

$ pytest-3
============================== test session starts ===============================
platform linux -- Python 3.9.18, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: /root/WOPI
collected 20 items

test/test_storageiface.py
....................                                                                                [100%]

Therefore this is ready to review.