Documentation: only python >=3.9 is supported?

[pfliu-nlp]

It seems that the client only supports python >=3.9, if yes,

• (1) we need to clarify this and,
• (2) we'd better document how to install python 3.9 WITH and WITHOUT sudo as well.

[neubig]

@pfliu-nlp : tests seem to be passing on version 3.8: https://github.com/neulab/explainaboard_client/actions/runs/3275693415/jobs/5390853373

Do you have a way to reproduce the error you were experiencing?

[neubig]

Closing, please re-open if it can be reproduced.