fixup! Use tags in docs building to split up requirements

docs/html/conf.py

@@ -13,14 +13,14 @@
 
 # -- General configuration ------------------------------------------------------------
 extensions = [
-    # extensions common to 'man' and 'html' builds
+    # extensions common to all builds
     "pip_sphinxext",
 ]
 
-# 'tags' is a special object handled by sphinx
+# 'tags' is a injected by sphinx
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#project-tags
-if "html" in tags or "READTHEDOCS" in os.environ:  # type: ignore[name-defined] # noqa: F821
-    # html specific deps
+if "man" not in tags:  # type: ignore[name-defined] # noqa: F821
+    # extensions not needed for building man pages
     extensions.extend(
         (
             # first-party extensions