Utilities/Sphinx: Restore explicit check for Sphinx 2.x or later

Refactoring in commit adbc8c982d (Utilities/Sphinx: Fix flake8 gripes in
cmake.py, 2023-03-13, v3.27.0-rc1~317^2) moved the assertion added by
commit cef51925a4 (Utilities/Sphinx: Require Sphinx 2.x or later,
2023-03-13, v3.27.0-rc1~317^2~2) to after imports, which is too late to
clearly reject older Sphinx versions.

Utilities/Sphinx/.flake8

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

Utilities/Sphinx/cmake.py

@@ -11,6 +11,11 @@ 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,)
+
 from docutils.utils.code_analyzer import Lexer, LexerError
 from docutils.parsers.rst import Directive, directives
 from docutils.transforms import Transform
@@ -110,9 +115,6 @@ CMakeLexer.tokens["root"] = [
 
 # %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-# Require at least Sphinx 2.x.
-assert sphinx.version_info >= (2,)
-
 logger = logging.getLogger(__name__)
 
 # RE to split multiple command signatures.