Utilities/Sphinx: Don't ignore flake8 E402

Refactor commit 1f39a3cd1a (Utilities/Sphinx: Restore explicit check for
Sphinx 2.x or later) to avoid needing to suppress flake8 E402. While
ignoring it with respect to the docutils/sphinx imports and the sphinx
version check was correct, the need to disable it for the whole file was
suboptimal.