Add supported python versions to documentation

[nikochiko]

Some python versions may not be supported by EvalAI-CLI.
The task is to install and test the app with various versions (python 3.x), test some functionality and check for any errors due to version, then update the readme correspondingly.

Only check for versions yet to reach end-of-life (reference: https://devguide.python.org/#status-of-python-branches)

Labels: [docs, good-first-issue]