Utilities/Sphinx: Don't ignore flake8 E402

Refactor !8555 (merged) 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. (Diff ignoring whitespace should help verify correctness.)

While we're at it, reorder some imports to better conform to what flake8 wants.

Backport: release