add support for MLP HP layer_sizes

[mfeurer]

Allow to upload lists of tuples as a hyperparameter value which is required for the MLPClassifier.

[codecov-io]

Codecov Report

Merging #879 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           develop    #879   +/-   ##
=======================================
  Coverage     88.4%   88.4%           
=======================================
  Files           37      37           
  Lines         4304    4304           
=======================================
  Hits          3805    3805           
  Misses         499     499

Impacted Files	Coverage Δ
openml/extensions/sklearn/extension.py	91.32% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1cfd6e...d37c9c4. Read the comment docs.