Add supported python versions to documentation

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]_

Start by reviewing the EvalAI-CLI README and the current Python branch status at the linked Python reference. Install the app with each supported, non-end-of-life Python 3.x version, exercise key functionality, and record any version-specific errors. Done means the README lists the verified supported versions and the checks pass or limitations are documented.

Written by the indexing model from the issue text.