Add support for Python3.14, drop support for 3.9 and 3.10

[tsmbland]

Description

Updates dependencies to use the latest versions of pandas, xarray, numpy and scipy. As a result, I've had to drop support for Python 3.9 and 3.10 as these are not supported by the latest package versions. There are also a few syntax changes related to pandas and xarray (in particular, xarray was warning about upcoming changes to some default parameters, so I've set these explicitly to avoid unwanted behaviour changes).

As shown by pandas 3.0, it's likely that any further major version updates to these packages will break things, so I've capped the upper versions to prevent unwanted breakages (e.g. when pandas 4 comes out). Not sure why I wasn't doing this already, to be honest.

Since this is a major breaking change, I'll wait for the next major release (i.e. v1.6.0) to merge this, whenever the time comes.

Fixes #825

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works