Migrate to Python 3.13

[MicahGale]

Description

Python 3.13 was released this Monday, which means that python 3.8 is no longer supported. By convention now OpenMC should only support python 3.11 - 3.13.

Alternatives

Do nothing and not support python 3.13. This is not clearly shown in the metadata.

Compatibility

Long term this will break backwards compatibility with previous python versions no longer supported.

[shimwell]

Keen to see python 3.13 support.

Perhaps it is worth making an openmc release before dropping support for python 3.8

[MicahGale]

That ship has sailed. OpenMC 0.15.0 requires Python 3.10.