DeprecationWarning with latest Sphinx: no_docstring is deprecated.

[mdickinson]

When running the Traits test suite with the latest Sphinx (3.5.1) from PyPI, I see the following DeprecationWarning:

/Users/mdickinson/Enthought/ETS/traits/traits/util/trait_documenter.py:80: RemovedInSphinx50Warning: The 'no_docstring' argument to TraitDocumenter.add_content() is deprecated.

[mdickinson]

Here's the relevant Sphinx PR: sphinx-doc/sphinx#8533

[mdickinson]

The fix may actually be very simple. The warning is coming from this code:

traits/traits/util/trait_documenter.py

Lines 78 to 80 in b3d815b

	def add_content(self, more_content, no_docstring=False):
	""" Never try to get a docstring from the trait."""
	ClassLevelDocumenter.add_content(self, more_content, no_docstring=True)

But I can't figure out why that override was necessary in the first place: if I simply remove that method, the documentation build seems to succeed in the normal way. My best guesses are (a) that the override was needed in some ancient version of Sphinx (this piece of the code is, after all, 9 years old), and/or (b) that Sphinx is or was attempting to look up the __doc__ attribute on CTrait objects. Up until recently, that __doc__ lookup would have returned None, potentially causing difficulties further down the line. But as of #1469, such a lookup would raise an AttributeError, and so would behave like any other Python object lacking a __doc__.

@itziakos Any insights? Do you remember what problem the add_content override was solving?