Numpy 2.0 compatability

[schlegelp]

This PR replaces a small number of numpy functions that have recently been dropped, namely np.row_stack and np.core.char.add.

I also escaped docstrings - currently they trigger a SyntaxWarning but would have caused a SyntaxError in the future.

With the above, morphops should work with modern numpy.

For reviewers

• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.

[vaipatel]

Thanks! Verified locally with NumPy 2.x, lgtm!

[vaipatel]

CI is failing only on the legacy Python 3.5 job, seems GitHub Actions can no longer provision Python 3.5 on the current runner. The PR changes were verified locally with NumPy 2.x, and the remaining 3.8/3.9 CI jobs pass. I’ll handle the CI matrix cleanup separately after merging.

[vaipatel]

@all-contributors please add @schlegelp for code

[allcontributors]

@vaipatel

I've put up a pull request to add @schlegelp! 🎉