Test failures with ase 3.26.0 due to missing `_get_constraints`

[sarahec]

ase 3.26 released on 2025-08-12 and is leading to broken tests.

FAILED tests/test_sinematrix.py::test_derivatives_numerical[eigenspectrum] - AttributeError: 'Atoms' object has no attribute '_get_constraints'.`

Found in
"tests/test_examples.py::test_examples"
"tests/test_general.py::test_atoms_to_system"
"tests/test_lmbtr.py"
"tests/test_mbtr.py"
"tests/test_sinematrix.py"
"tests/test_valle_oganov.py"