Allow docs to build without PyQt installed.

PyQt/PySide is needed to list the config options for the Qt console. But
some environments and contributors may not have it, and it's not always
easy to install. This prints a warning and skips the list of Qt console
options if the import fails.
Merge pull request #4642 from takluyver/docs-build-noqt

Allow docs to build without PyQt installed.