Formatted xcube Python code using black

[forman]

• Reformatted xcube code base including Jupyter NBs using black default settings. It implies a line length of 88 characters and double quotes for string literals.
• Added .editorconfig for IDEs not recognising black's defaults.
• Added black code style badge to README.md

NOTE: I'll prepare a separate PR for the dev guide update, so it is easier to review for the whole dev team.

Checklist:

• [ ] ~Add unit tests and/or doctests in docstrings~
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [ ] ~New/modified features documented in docs/source/*~ (see above)
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.39184% with 128 lines in your changes are missing coverage. Please review.

Project coverage is 93.21%. Comparing base (ac43633) to head (b9a38b9).

Files	Patch %	Lines
test/core/gen2/remote/server.py	0.00%	47 Missing :warning:
test/core/gen2/remote/test_service.py	15.38%	33 Missing :warning:
test/cli/test_benchmark.py	5.55%	17 Missing :warning:
test/core/store/ref/test_store.py	45.00%	11 Missing :warning:
test/core/diagnosticstore.py	76.19%	5 Missing :warning:
test/cli/test_prune.py	88.88%	4 Missing :warning:
test/core/test_dsio.py	96.70%	3 Missing :warning:
...st/core/byoa/test_data/user_code/impl/algorithm.py	33.33%	2 Missing :warning:
test/core/test_chunkstore.py	91.30%	2 Missing :warning:
examples/serve/demo/resample_in_time.py	75.00%	1 Missing :warning:
... and 3 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #948 +/- ## ======================================= Coverage 93.21% 93.21% ======================================= Files 458 458 Lines 35812 35812 ======================================= Hits 33382 33382 Misses 2430 2430 ```

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