xplt 0.10: Update dependencies and require python >=3.9

[eltos]

This is for release 0.10

From https://github.com/xsuite/xplt/pull/9#issuecomment-2350899319

I also added a requirement for the version of pint as users reported that that xplt stopped working after upgrading to numpy 2.0 (even reinstalling xplt did not help as pint was not updated).

Present version of pint does not support python 3.8 so I had to remove it from the test workflow.

TODO's:

• The update of pint introduced an issue which went unnoticed
  • [x] Update CI to check for this
  • [x] Update code to work with new pint version