Avoid future breakage of setup.py invokations

See #8659.

Author: astrojuanlu

readthedocs/doc_builder/python_environments.py

@@ -197,7 +197,10 @@ def install_build_tools(self):
                     'install',
                     '-U',
                     'virtualenv',
-                    'setuptools',
+                    # We cap setuptools to avoid breakage of projects
+                    # relying on setup.py invokations,
+                    # see https://github.com/readthedocs/readthedocs.org/issues/8659
+                    '"setuptools<58.3.0"',
                 ]
                 self.build_env.run(
                     *cmd,