global: remove Python 2 code

.github/workflows/tests.yml

docs/conf.py

invenio_previewer/__init__.py

invenio_previewer/_compat.py

invenio_previewer/api.py

invenio_previewer/ext.py

invenio_previewer/extensions/csv_dthreejs.py

invenio_previewer/extensions/default.py

invenio_previewer/extensions/json_prismjs.py

invenio_previewer/extensions/pdfjs.py

invenio_previewer/extensions/simple_image.py

invenio_previewer/extensions/xml_prismjs.py

invenio_previewer/extensions/zip.py

invenio_previewer/proxies.py

invenio_previewer/version.py

invenio_previewer/views.py

invenio_previewer/webpack.py

tests/conftest.py

tests/test_invenio_previewer.py

tests/test_macros.py