Add support for serializing numpy data types.

[PGijsbers]

Notes on the fix in #304.

[codecov-io]

Codecov Report

Merging #635 into develop will decrease coverage by 0.26%.
The diff coverage is 100%.

@@ Coverage Diff @@## develop #635 +/- ##
===========================================
- Coverage 90.12% 89.86% -0.27% 
===========================================
Files 32 32 Lines 3232 3236 +4 ===========================================
- Hits 2913 2908 -5 - Misses 319 328 +9

Impacted Files	Coverage Δ
openml/flows/sklearn_converter.py	90.48% <100%> (+0.08%)	⬆️
openml/_api_calls.py	84.41% <0%> (-3.9%)	⬇️
openml/tasks/functions.py	86.45% <0%> (-1.94%)	⬇️
openml/runs/functions.py	86.4% <0%> (-0.66%)	⬇️

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[mfeurer]

Looks good, could you please add a unit test ensuring the behavior and also add a check that numpy arrays cannot be serialized?

[PGijsbers]

@mfeurer I assumed you would get a ping automatically, but I am not sure if this is the case. I added the tests.

[mfeurer]

I get a ping for a commit, but I usually do not check the commits because I don't know if that commit makes it ready for review again. Will check now.