MNT: Remove np.float_ alias; it is removed in NumPy 2.0

[larrybradley]

Description

Importing scikit-image in an environment with numpy 2.0dev gives an AttributeError:

AttributeError: `np.float_` was removed in the NumPy 2.0 release. Use `np.float64` instead.. Did you mean: 'float16'?

xref: numpy/numpy#24376

This PR removes the usage of the np.float_ alias. It's the same as np.float64.

>>> np.float_ is np.float64
True

Checklist

• A descriptive but concise pull request title
• Docstrings for all functions
• Unit tests
• A gallery example in ./doc/examples for new features
• Contribution guide is followed

Release note

Summarize the introduced changes in the code block below in one or a few sentences. The
summary will be included in the next release notes automatically:

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