Fixed a NumPy 2.x testing issue and added a NumPy 2.x build to the test workflow

[swryan]

Summary

Tests in dymos/examples/brachistochrone/test/test_duplicate_constraints.py were failing with NumPy 2.x due to a change in the numpy representation:

AssertionError: 'Dupl[71 chars] `theta` are used in multiple path constraints:\n{np.int64(0)}' != 'Dupl[71 chars] `theta` are used in multiple path constraints:\n{0}'
  Duplicate constraint in phase traj0.phases.phase0. The following indices of `theta` are used in multiple path constraints:
- {np.int64(0)}
+ {0}

The generation of that error message has been wrapped in a NumPy printoptions context to preserve the previous formatting.

Also:

• a NumPy 2.x build has been added to the test workflow

Related Issues

• Resolves #

Backwards incompatibilities

None

New Dependencies

None

[coveralls]

coverage: 93.695% (-0.2%) from 93.903%
when pulling f1e53eb on swryan:numpy2_tests
into 6b48528 on OpenMDAO:master.