Change 'Development Status :: 3 - Alpha' in setup.py

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Development Status 3 - Alpha

as currently used Development Status :: 3 - Alpha in: https://salsa.debian.org/reproducible-builds/diffoscope/-/blob/93b4806f877c81e2b792a5808d642b20b2e7cbd7/setup.py#L74-75

    classifiers=[
        'Development Status :: 3 - Alpha',

more accurate status is: Development Status :: 5 - Production/Stable or Development Status :: 6 - Mature