Only use sphinx-build-compatibility on Read the Docs

Author: adamchainz

docs/conf.py

@@ -5,6 +5,7 @@
 # https://www.sphinx-doc.org/en/master/usage/configuration.html
 from __future__ import annotations
 
+import os
 import sys
 from pathlib import Path
 
@@ -39,9 +40,10 @@
     "sphinx.ext.autodoc",
     "sphinx.ext.viewcode",
     "sphinx.ext.intersphinx",
-    "sphinx_build_compatibility.extension",
     "sphinx_copybutton",
 ]
+if os.environ.get("READTHEDOCS") == "True":
+    extensions.append("sphinx_build_compatibility.extension")
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.