deps: Make Sphinx optional

Signed-off-by: Ryan Northey <[email protected]>

Author: phlax

Diff for: pyproject.toml

@@ -39,9 +39,6 @@ classifiers = [
     "Topic :: Text Processing",
     "Topic :: Utilities",
 ]
-dependencies = [
-    "Sphinx>=5",
-]
 dynamic = ["version"]
 
 [project.optional-dependencies]
@@ -73,3 +70,8 @@ include = [
 
 [tool.mypy]
 ignore_missing_imports = true
+
+[project.optional-dependencies]
+standalone = [
+  "sphinx > 5",
+]