CI: Add Python 3.14 to test matrix

[EwoutH]

Add Python 3.14 to the test matrix of the main GitHub Actions workflow.

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

Thanks!