avoid crash in `eessi check`: update `get_repo_attribute` to take int…

…o account that attribute value can be 0 byte value (empty) (PR #13)

- use custom help option to control its placement and behaviour (PR #7)

- initial implementation of eessi check (PR #10)
- update CI workflow to test eessi check (PR #11)

- require that EESSI version to use is specified in eessi shell via -…

…-eessi-version option (PR #3)

- force interactive shell for eessi shell + add tests for eessi shell in CI workflow (PR #4)
- Add option to display version and show help with no options (PR #5)

- initial implementation for eessi shell

- repackage with modern PyPA standards (#2)

rename command to eessi

re-design using Typer, add placeholders for 'check' and 'shell' subco…

…mmands

initial release, with (very) basic eessi-cli init command (#1)