Fix securedrop-admin crash when no operation given under Python 3

[rmol]

Status

Ready for review

Description of Changes

towards #4910.

Under Python 3, running securedrop-admin with no positional argument results in an ugly error, due to https://bugs.python.org/issue16308.

Under Python 3.7, we could simply add dest and required arguments to the add_subparsers call, but required isn't available in Python 3.5, so would break under Tails 3.

Testing

Run securedrop/bin/dev-shell bash to start a shell in the dev container, then run:

• cd ..
• ./securedrop-admin setup
• ./securedrop-admin -v

You should see Please specify an operation. followed by the securedrop-admin help.

Deployment

This only affects the admin workstation.

Checklist

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

[kushaldas]

This is good. Approved.

[emkll]

Thanks @rmol went through the fix and does what's advertised, ./securedrop-admin now works and prints the help page, as expected.

I've also edited the PR description to say that it doesn't completely resolve #4910 : while it allows a user to run ./securedrop-admin with no argument, it does not address the issue that the venv in admin/.ven3 needs to be destroyed/rebuilt for Tails 4. We should, as part of #4910 either

1. Provide docs for the 1.0.0 -> 1.1.0 release to indicate that the venv needs to be destroyed and rebuilt
2. Automate the process (this might be difficult as we cannot do this as part of the updater code, we would need to either create a new ./securedrop-admin option or modify ./securedrop-admin setup (check if tails3 and python version in venv is 3.5)