python: elements of __all__ must be strings

[daniloegea]

Description

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

LGTM

According to Python's reference:

If the list of identifiers is replaced by a star ('*'), all public names defined in the module are bound in the local namespace for the scope where the import statement occurs.

The public names defined by a module are determined by checking the module’s namespace for a variable named all; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in all are all considered public and are required to exist. If all is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character ('_'). all should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).