v0.20.20

MLJBase v0.20.20

Diff since v0.20.19

Closed issues:

• Issue in serializable (#843)
• allow train_test_pairs to work on generic vector y? (#847)

Merged pull requests:

• [Patch for older 0.19 series] Fix scitype checks for Annotator model types (such as detectors) (#842) (@ablaom)
• Relax type restriction on y in train_test_pairs(::StratifiedCV, rows, y) (#848) (@ablaom)
• For a 0.20.20 release (#849) (@ablaom)