fix urljoin for arbitrary URL protocols

[rmorshea]

This works around an issue with urllib.parse.urljoin where it only handles relative URLs for protocols contained in urllib.parse.uses_relative. As it happens common protocols used with git, like ssh or git+ssh are not in that list.

For more info: #6499 (comment)

Pull Request Check List

Resolves: #6499

• Added tests for changed code.
• Updated documentation for changed code.

[dimbleby]

I can't tell what this fixes, a test case showing the point would be helpful

[rmorshea]

@dimbleby, I'll add a test, but for now, the problem this solves is described here.

[rmorshea]

@dimbleby, I've added tests. Doing so required a bit of refactoring to make testing easier since actually cloning the test repo via SSH would require significant effort without a correspondingly large benefit. The core the of the change, as before, is urlpathjoin, which appropriately handles relative paths for URLs with ssh:// schemas.

[github-actions]

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