requirements prevent use of Sphinx 3.1.0

[jmckenna]

• Sphinx 3.1.0 has been released today (https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
• currently breathe requirements are forced to be Sphinx>=3.0,<3.1 (related to recent change through strictly depend on sphinx's minor version #498)
• suggest to bump up the Sphinx version requirements

[jakobandersen]

If I remember correctly there should be no Sphinx changes to prevent a bump to Sphinx>=3.0,<3.2.
There may be cases in the documentation that could benefit from

• C, added scope control directives, :rst:dir:c:namespace,
  :rst:dir:c:namespace-push, and :rst:dir:c:namespace-pop.

Perhaps we should open an issue accumulating TODOs for an overview of when it may be beneficial to raise to lower Sphinx version bound.

[vermeeren]

@jmckenna Fixed in breathe v4.19.1, just released it. Thanks for the issue.

@jakobandersen Yeah I think that would be a good idea, we could also document it in some COMPATIBILITY.md file in the repository, that might be easier to track long-term. I am fine with both cases.

(Appears the Sphinx branch 3.0.x has been removed, will update CICD in a sec. Edit: 75d0803)

[jmckenna]

Verified here. Thanks for the quick fix @vermeeren