Fix import recording warning

Modified find_module to be compatible with find_spec. Fixes this warning:

tests/test_Config.py::ConfigTest::testBadImports
tests/test_Config.py::ConfigTest::testImports
  <frozen importlib._bootstrap>:914: ImportWarning: RecordingImporter.find_spec() not found; falling back to find_module()

python/lsst/pex/config/config.py

@@ -894,7 +894,7 @@ def uninstall(self):
         """Uninstall the importer."""
         sys.meta_path = self.origMetaPath
 
-    def find_module(self, fullname, path=None):
+    def find_spec(self, fullname, path, target=None):
         """Find a module.
 
         Called as part of the ``import`` chain of events.