add svg_link_prefix option to inheritance_diagram

[eteq]

Subject: Add an option to set the prefix for links in SVG inheritance diagrams

Feature or Bugfix

• Feature

Purpose

• Provide a way for extensions or projects that embed the inheritance diagram SVG in pages to have the links work.

Detail

As discussed in astropy/astropy#4935 and #2484, the inheritance diagram machinery currently yields broken links as we implemented it in sphinx-automodapi and astropy. You can see an example here: https://docs.astropy.org/en/v3.1.2/coordinates/index.html#class-inheritance-diagram . The gist of the problem (as discussed in #2484) is that we embed the SVG in the page. This means the path to the actual class pages don't have the '..' that apparently gets created when embedding as a separate file. (FWIW, the links originally worked when we first did this, but then the changes in #967 ended up breaking it. And here we are years later with me finally having realized the underlying cause!).

So this PR introduces a new config option in inheritance_diagram that sets the prefix. The default is still ../, so it shouldn't break other code, but it will then be possible to supress the .. in cases like ours where it's breaking the links. I think it may partly address #3176 by giving that use case the flexibility to pick their own prefix as well (although I'm not 100% sure about that one).

Relates

• Fix link target path for SVG inheritance diagrams #2484
• Broken links in inheritance diagrams #3176
• Sphinx class inheritance diagrams not linking classes correctly astropy/astropy#4935

[eteq]

Also, I wasn't entirely sure if this was supposed to be against the 2.1.1 or the master branch - I'm not clear on whether adding new configuration items is considered backwards-compatibility breaking or not, so using master seemed like the best choice. I can re-submit it against the 2.1.1 branch if I mis-interpreted that, though.

[codecov]

Codecov Report

Merging #6454 into master will decrease coverage by <.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #6454      +/-   ##
==========================================
- Coverage   84.22%   84.22%   -0.01%     
==========================================
  Files         280      277       -3     
  Lines       39942    39890      -52     
  Branches     5899     5897       -2     
==========================================
- Hits        33643    33597      -46     
+ Misses       4989     4985       -4     
+ Partials     1310     1308       -2

Impacted Files	Coverage Δ
sphinx/ext/inheritance_diagram.py	81.69% <60%> (+0.16%)	⬆️
sphinx/writers/__init__.py
tests/roots/test-setup/setup.py
tests/roots/test-setup/doc/conf.py

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[tk0miya]

What case are users need to change this? If my understanding correct, this is only used for automodapi extension. If so, I propose you to modify node['refuri'] for SVG images on automodapi side.
Thanks,

[tk0miya]

I'm closing this because main usecase of this PR is not clear.
Please feel free to reopen if you'd like to merge this again.
Thanks,