Remove stale info from flags docstring

[keckler]

What is the change?

The linked bitbucket site no longer appears to exist. It has been migrated to Github, but the associated ticket appears to have been lost without any apparent resolution.

https://github.com/ethanfurman/aenum

Why is the change being made?

Removing stale info.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.