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.

Utilities/Sphinx/.flake8

@@ -1,3 +0,0 @@
-[flake8]
-per-file-ignores =
-     cmake.py: E402

Utilities/Sphinx/cmake.py

@@ -11,11 +11,8 @@ from typing import Any, List, Tuple, Type, cast
 
 import sphinx
 
-# Require at least Sphinx 2.x.
-# flake8 issues E402 for imports after this, but the purpose of this
-# check is to fail more clearly if the imports below will fail.
-assert sphinx.version_info >= (2,)
-
+# The following imports may fail if we don't have Sphinx 2.x or later.
+if sphinx.version_info >= (2,):
     from docutils.utils.code_analyzer import Lexer, LexerError
     from docutils.parsers.rst import Directive, directives
     from docutils.transforms import Transform
@@ -29,6 +26,9 @@ from sphinx.util.docutils import ReferenceRole
     from sphinx.util.nodes import make_refnode
     from sphinx.util import logging, ws_re
     from sphinx import addnodes
+else:
+    # Sphinx 2.x is required.
+    assert sphinx.version_info >= (2,)
 
 # END imports