Clean up compatibility code for older Sphinx versions

[mdickinson]

There's some compatibility code in the trait_documenter tests for dealing with Sphinx versions older than 2.1. The primary reason for this code was that at the time, the most recent Sphinx release available from the buildsystem was 1.8.

We now have a newer version of Sphinx in the buildsystem, and this compatibility code can be removed.

Code link:

traits/traits/util/tests/test_trait_documenter.py

Lines 155 to 164 in 2859f76

	# Backwards compatibility hack: for now, we need to be compatible
	# with both Sphinx < 2.1 (whose DocumenterBridge doesn't take
	# a state argument) and Sphinx >= 2.3 (which warns if the state
	# isn't passed). Eventually we should be able to drop support
	# for Sphinx < 2.1.
	kwds = {}
	if sphinx.version_info >= (2, 1):
	state = mock.Mock()
	state.document.settings.tab_width = 8
	kwds["state"] = state