MAINT explain the reason for conftest.py in the root folder
+# This file is here so that when running from the root folder
+# ./sklearn is added to sys.path by pytest.
+# See https://docs.pytest.org/en/latest/pythonpath.html for more details.
+# For example, this allows to build extensions in place and run pytest
+# doc/modules/clustering.rst and use sklearn from the local folder
+# rather than the one from site-packages.
