BUG: xarray.backends.plugins.get_backend: fix passing in a backend directly

[FRidh]

The open_dataset and open_mfdataset functions allow a user to pass
in a (custom) backend directly. This was however not functioning.

Selecting an engine with a string was functioning as expected, however,
passing in a backend class directly failed.

An instance of the backend is now created. A test has been added, as
well as some type hints.

• Closes #xxxx
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[pep8speaks]

Hello @FRidh! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-09-20 09:53:27 UTC

[FRidh]

readthedocs failure seems to be unrelated judging also from other PR's.

[github-actions]

Unit Test Results

6 files 6 suites 55m 0s ⏱️
16 229 tests 14 493 ✔️ 1 735 💤 1 ❌
90 570 runs 82 390 ✔️ 8 174 💤 6 ❌

For more details on these failures, see this check.

Results for commit 628fbc8.

[FRidh]

looks like there is a similar test in test_backends_api.py. Probably did something wrong myself so this is not needed.

[keewis]

see #5684: you're not wrong in assuming that there's a bug in the backend code, but we have been a bit slow in merging the PR fixing it.

[Illviljan]

The typing here is nice though if you want to continue with that.