Replace numpy dtype by builtin dtype

[sciunto]

Description

Follows #5103

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[grlee77]

I see an unrelated sphinx warning is causing some failures, but this fix looks good and uncontroversial so I will go ahead and merge to help get CI toward a working state again.

Warning, treated as error:
role 'arxiv' is already registered, it will be overridden
make[1]: *** [Makefile:58: html] Error 2
make[1]: Leaving directory '/home/runner/work/scikit-image/scikit-image/doc'

[hmaarrfk]

Should we backport this. Seems safe

[sciunto]

I see an unrelated sphinx warning is causing some failures, but this fix looks good and uncontroversial so I will go ahead and merge to help get CI toward a working state again.

Warning, treated as error:
role 'arxiv' is already registered, it will be overridden
make[1]: *** [Makefile:58: html] Error 2
make[1]: Leaving directory '/home/runner/work/scikit-image/scikit-image/doc'

Oh, it's related to a feature I added. I guess it is implemented in sphinx now. I'll have a look.