add __all__ to declare public API

[gotmax23]

Before

$ pydoc trove_classifiers
Help on package trove_classifiers:

NAME
    trove_classifiers

PACKAGE CONTENTS
    __main__

DATA
    Dict = typing.Dict
        A generic version of dict.

    List = typing.List
        A generic version of list.

    Set = typing.Set
        A generic version of set.

    __annotations__ = {'all_classifiers': typing.List[str], 'classifiers':...
    all_classifiers = ['Development Status :: 1 - Planning', 'Development ...
    classifiers = {'Development Status :: 1 - Planning', 'Development Stat...
    deprecated_classifiers = {'Framework :: Django CMS :: 4.2': ['Framewor...
    sorted_classifiers = ['Development Status :: 1 - Planning', 'Developme...

After

$ pydoc trove_classifiers
Help on package trove_classifiers:

NAME
    trove_classifiers

PACKAGE CONTENTS
    __main__

DATA
    __all__ = ['all_classifiers', 'deprecated_classifiers', 'sorted_classi...
    __annotations__ = {'all_classifiers': typing.List[str], 'classifiers':...
    all_classifiers = ['Development Status :: 1 - Planning', 'Development ...
    deprecated_classifiers = {'Framework :: Django CMS :: 4.2': ['Framewor...
    sorted_classifiers = ['Development Status :: 1 - Planning', 'Developme...

[gotmax23]

Thanks!