Change default docker image to py37

[Didayolo]

Change the default docker image from codalab/codalab-legacy:py3 to codalab/codalab-legacy:py37.

• This way it is the same as on CodaLab Competitions, so bundles using the default image that work on CodaLab will work on Codabench
• The py37 image is documented on https://github.com/codalab/codalab-dockers
• We should not change this in the future to avoid breaking bundles
• It is using decent libraries