Update documentation

[mfeurer]

This pull request does

• dynamically generate the copyright year
• replaces pprint by print
• updates one example to use a more common task on the test server
• fix a few docstring syntax issues
• introduce stricter checks for correct XML files (I had old files on my machine, and the error message were not helpful).

[Neeratyoy]

Looks like a certain test__get_cached_dataset unit test is failing since the new nominal values check in openml/datasets/data_feature.py is raising an error.

[codecov-io]

Codecov Report

Merging #740 into develop will decrease coverage by 0.23%.
The diff coverage is 55.55%.

@@ Coverage Diff @@## develop #740 +/- ##
===========================================
- Coverage 87.99% 87.76% -0.24% 
===========================================
Files 36 36 Lines 4081 4004 -77 ===========================================
- Hits 3591 3514 -77 
Misses 490 490

Impacted Files	Coverage Δ
openml/study/functions.py	88.23% <ø> (ø)	⬆️
openml/datasets/dataset.py	84.07% <ø> (ø)	⬆️
openml/setups/setup.py	42.3% <ø> (ø)	⬆️
openml/testing.py	96.21% <0%> (-0.18%)	⬇️
openml/tasks/functions.py	84.93% <100%> (ø)	⬆️
openml/datasets/data_feature.py	68% <57.14%> (-2%)	⬇️
openml/extensions/sklearn/extension.py	90.62% <0%> (-0.59%)	⬇️
openml/datasets/functions.py	95.16% <0%> (-0.33%)	⬇️
openml/flows/functions.py	87.78% <0%> (-0.19%)	⬇️
openml/flows/flow.py	93.8% <0%> (-0.06%)	⬇️
... and 3 more

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