Dropping Python support for versions < 3.10

[clelange]

Given that Python 3.9 will be EOL next month, we can at some point consider dropping support for Python versions older than 3.10.
See also conda-forge/hepdata-lib-feedstock#5 (comment)

For now, I think we can just keep the support but should this affect future developments, I think it's reasonable to drop it. We should probably check when the Python version on LXPLUS gets updated to 3.10.

[GraemeWatt]

I agree that it makes sense to continue supporting older Python versions where possible to encourage wider use of hepdata_lib, as long as these older Python versions are still being tested in the CI. One issue (HEPData/hepdata-validator#69) I had is that I dropped Python 3.6 support two years ago in v0.3.4 of the hepdata-validator, but then realised that Python 3.6 support was still needed by hepdata_lib, so I reinstated it in v0.3.5. Earlier this year I dropped hepdata-validator CI testing for Python 3.6 as it stopped being easily available in the GitHub Actions runner images (HEPData/hepdata-validator#68), without using something like Micromamba as done by hepdata_lib. So I'd probably prefer hepdata_lib to drop Python 3.6 support, retaining support for Python 3.7 and later, then I could make the same change in a future release of the hepdata-validator.