Lazily load data_dir into the top-level namespace

[grlee77]

Description

This is a follow up PR to #5101 that delays loading of skimage.data_dir. Currently, importing that in the top-level __init__.py results in importing Pooch. With the change here, data_dir is lazily loaded.

The diff here is much smaller than it appears because all code in _fetchers.py is just the old __init__.py. Aside from removing __all__ after the rename, I made only a 1 line change to fix an issue with parsing the modified __version__ string containing the git hash introduced in #5909.

This PR also fixes an issue where skimage.data.vortex was missing from skimage.data.__all__.

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
• Descriptive commit messages (see below)

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.

[pep8speaks]

Hello @grlee77! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage/data/_fetchers.py:

Line 439:80: E501 line too long (98 > 79 characters)
Line 480:80: E501 line too long (81 > 79 characters)
Line 506:80: E501 line too long (578 > 79 characters)
Line 511:80: E501 line too long (84 > 79 characters)
Line 555:80: E501 line too long (98 > 79 characters)
Line 566:80: E501 line too long (81 > 79 characters)
Line 963:80: E501 line too long (99 > 79 characters)
Line 969:80: E501 line too long (80 > 79 characters)

Comment last updated at 2021-10-29 20:08:35 UTC

[grlee77]

This lazy loading should also help resolve #5146 (at least the error would not occur upon the top-level import). I will add another commit here to deal with the issue of missing pooch.__version__ on older Pooch (as reported in #5146 (comment))

[grlee77]

I manually verified that 26a580e avoids the problem with AttributeError: module 'pooch' has no attribute '__version__' with Pooch 1.1.0 since we do not run CI tests with that version.

[grlee77]

closes #5146

[alexdesiqueira]

Thank you @grlee77!

[imagesc-bot]

This pull request has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/why-does-init-py-attempt-to-download-sample-data/79665/4