Adapt to newest Sphinx version

[jakobandersen]

It looks like the CI was testing against old, unsupported Sphinx versions. Generating the documentation results in a lot of warnings that were not there before:

• Duplicate declarations. This is not a new problem, but now there are warnings about it. See Sphinx with breathe throws warnings when building. sphinx-doc/sphinx#4820 and C++, add role/directive for rendering aliases of declarations. sphinx-doc/sphinx#4981. When the Sphinx support for aliases is implemented I suggest that Breathe directives get an :alias: for specifying that no declarations should be inserted, but only aliases.
• Error in expression: int bar(int n, int a[static n]). Also a warning about an old problem.

Fixes #360
See also #374 that will be fixed when upgrading the Sphinx version.

[jakobandersen]

I think the PR is ready now.

[vermeeren]

Thanks for updating the tests, merged.