Stop using git:// for submodules

[peterbell10]

Description

Trying to clone scikit-image with --recursive fails with the following error:

fatal: remote error: 
  The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.

Following the linked blog post you can see that support for git:// was disabled as of yesterday, the 15th of March.

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.
• There is a bot to help automate backporting a PR to an older branch. For
  example, to backport to v0.19.x after merging, add the following in a PR
  comment: @meeseeksdev backport to v0.19.x
• To run benchmarks on a PR, add the run-benchmark label. To rerun, the label
  can be removed and then added again. The benchmark output can be checked in
  the "Actions" tab.

[hmaarrfk]

@scikit-image/core can this file be removed entirely?

[mkcor]

@hmaarrfk I guess it's needed because it provides https://scikit-image.org/docs/stable/gitwash/index.html (that https://scikit-image.org/docs/stable/contribute.html links to).

Related to #6180.

[grlee77]

Thanks @peterbell10!

[rfezzani]

Can we backport this to v0.18 to offer a solution to #6510? (see #6510 (comment))

[grlee77]

Can we backport this to v0.18 to offer a solution to #6510? (see #6510 (comment))

Do you mean just backport the commit or do an actual 0.18.x release? Just adding the commit to that branch seems okay, but I don't think we should make new releases from that branch at this point.

[rfezzani]

I agree, a new release is absolutely not necessary. I think backporting the commit suffices ;).

[rfezzani]

BTW, adding this commit to all the branch version gives the users the ability to install any version using

pip install git+https://github.com/scikit-image/scikit-image.git@v0.X.Y

which can be interesting in backward compatibility issues like #6510