CI: catch unexpected warning messages

The CI runs of sherpa#1752 after
pulling in this branch sherpa#1753
started to fail because of a warning message that I could not
replicate. A search lead to this set of issues

- pypa/setuptools#2104
- pypa/setuptools#2052
- pypa/setuptools#1383

which suggests there's a subtle interaction between pip and
setuptools that we are falling over. As we currently have setuptools
pinned to < 60, whilst the distutils changes coming in Python 3.12
work through the ecosystem, the OTS code we run is likely to
hit this issue (i.e. nothing we can fix) so we can just skip the
warnings. Hopefully we can remove these additions soon.

sherpa/conftest.py

@@ -1,5 +1,5 @@
 #
-#  Copyright (C) 2016, 2017, 2018, 2019, 2020, 2021, 2022
+#  Copyright (C) 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023
 #  Smithsonian Astrophysical Observatory
 #
 #
@@ -188,6 +188,19 @@ def pytest_collection_modifyitems(config, items):
         ],
     VisibleDeprecationWarning:
         [],
+    ImportWarning:
+    [
+        # It is not clear why this happens - seen when testing
+        # https://github.com/sherpa/sherpa/pull/1752 - and the problem appears
+        # to be a setuptools/pip interaction, based on
+        #   https://github.com/pypa/setuptools/issues/2104
+        #   https://github.com/pypa/setuptools/issues/2052
+        #   https://github.com/pypa/setuptools/issues/1383
+        # At present Sherpa forces an old version of setuptoools (<60) so it is
+        # unlikely to be fixed. When the setuptools restriction is removed we can
+        # hopefully remove this.
+        r"VendorImporter.find_spec\(\) not found; falling back to find_module\(\)",
+    ]
 }
 
 if have_astropy: