Deprecation: Fix ptp --> np.ptp for numpy 2.0.0

[CSSFrancis]

Description of the change

Support numpy 2.0.0

Progress of the PR

• Pass tests with numpy 2.0.0

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in CHANGELOG.rst.
• Contributor(s) are listed correctly in credits in diffsims/release_info.py and
  in .zenodo.json.

[CSSFrancis]

@hakonanes If you want to review/merge this I can start the process of making a numpy 2.0.0 patch release.

[hakonanes]

Instead of repeating this three places, I suggest to add a private diffsims.constants module like we've done in orix: https://github.com/pyxem/orix/blob/develop/orix/constants.py. We can then import this warning from diffsims.constants import VisibleDeprecationWarning.

[hakonanes]

Two minor suggestions. Not required for merging.