Drop support for Python 3.6 or re-add tests in CI

[GraemeWatt]

See PR #68. I've removed the Python 3.6 tests from the CI because it's no longer provided in the runner images provided by GitHub Actions, but I've kept python_requires='>=3.6' in the setup.py file, because the hepdata_lib testing requires a version of the hepdata-validator compatible with Python 3.6. The hepdata_lib testing uses Micromamba for setting up Python 3.6 rather than relying on the runner images provided in GitHub Actions. We should either drop Python 3.6 support in the hepdata-validator (e.g. after hepdata_lib drops support) or re-add testing for Python 3.6 (e.g. using Micromamba).