test: Add Python 3.15 beta 4 to test matrix in GitHub Actions

[cclauss]

As recommended at:

• improve tests for python 3.14 #3906 (comment)
• https://www.python.org/downloads/release/python-3150b4/

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• Include unit tests and update documentation for new features.

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• fix: Bug fix
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[cclauss]

How do we get the final test to run?

altair/.github/workflows/tests.yml

Lines 97 to 108 in 5cf3b90

	# Branch protection requires one stable check name; this aggregates the test matrix.
	check-all-tests:
	name: Check that all tests passed
	runs-on: ubuntu-latest
	needs: test
	if: ${{ always() }}
	steps:
	- name: Check matrix result
	run: |
	if [ "${{ needs.test.result }}" != "success" ]; then
	exit 1
	fi

[mattijn]

I see:

  × Failed to build `pyproj==3.7.2`

Which is a dependency of geopandas, so maybe geopandas has no support yet for Python 3.15? We may skip geopandas related tests for now.

[cclauss]

• Add Python 3.14 to the testing pyproj4/pyproj#1608
• Add Python 3.15 beta 4 to the testing geopandas/geopandas#3814