pass skipna=False explicitly

[TwsThomas]

What does this PR implement/fix? Explain your changes.

Pass skipna=False explicitly when calling pd.api.types.infer_dtype to avoid behavioral change and pandas FutureWarning: " A future version of pandas will default to skipna=True. To silence this warning, pass skipna=True|False explicitly."

[codecov-io]

Codecov Report

Merging #766 into develop will decrease coverage by 0.09%.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #766 +/- ##
==========================================
- Coverage 87.67% 87.57% -0.1% 
==========================================
Files 36 36 Lines 4072 4072 ==========================================
- Hits 3570 3566 -4 - Misses 502 506 +4

Impacted Files	Coverage Δ
openml/datasets/functions.py	95.48% <ø> (ø)	⬆️
openml/exceptions.py	84.37% <0%> (-9.38%)	⬇️
openml/runs/functions.py	81.79% <0%> (-0.29%)	⬇️

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[mfeurer]

Thanks a lot!