Update data explorer to run on Python 3

[wnojopra]

Tested by running locally with amp_pd, 1000_genomes.

TODO:

• In dataset_config/framingham_heart_study_teaching/ui.json search_placecholder_text, do you mind making "bmi" and "stroke" lowercase?
• Fix/unit test 1000 Genomes covered 20x or greater facet

[wnojopra]

@melissachang Could you please check if the branch protection setting needs to be changed in this repo as well? (via https://support.circleci.com/hc/en-us/articles/360004346254-Workflow-status-checks-never-completes-because-of-ci-circleci-Waiting-for-status-to-be-reported) Thanks!

[melissachang]

Updated. Every once in a while, can you redeploy 1000 Genomes and make sure things still work? Thanks.

[wnojopra]

Sure thing, I'll redeploy soon. Thanks!