Drop Python 3.6 support

[shimizukawa]

because...

• 2021-12-23, 3.6 has reached the end-of-life https://peps.python.org/pep-0494/#lifespan
• ubuntu-latest (22.04) doesn't support python-3.6 as: https://github.com/sphinx-doc/sphinx-intl/actions/runs/4090490381/jobs/7053940455

[shimizukawa]

This PR will be merged to release 2.1.0.