setup: update classifiers for Python

[topless]

In setup.py of every invenio module we need to update classifiers when we drop support for Python2, I have also added a few more that make sense, and a list of those that need to be reviewed.

I believe it would make sense to automatically update all classifiers among all repositories when ready.

• [ ] Development Status should remain the as defined per project
• [ ] review the license
• [ ] We can request from pypi Framework :: Invenio like Framework :: Django

classifiers=[
    'Development Status :: 1 - Planning',
    'Environment :: Web Environment',
    'Intended Audience :: Developers',
    'License :: OSI Approved :: MIT License',
    'Operating System :: OS Independent',
    'Programming Language :: Python :: 3.6',
    'Programming Language :: Python :: 3.7',
    'Programming Language :: Python :: 3',
    'Programming Language :: Python',
    'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
    'Topic :: Software Development :: Libraries :: Python Modules',
],