TST Fix test_conversion_missing (#341)

This test is failing in CIs in #340.

I think it is because there is no warning raised (not sure why the
`UserWarning` is there). It used to pass because of this bug in pytest
pytest-dev/pytest#9036, but this was fixed in
pytest-dev/pytest#11129 and included in a recent
release, thus our CIs are failing.

Note that I have update the test to be more specific about the exception
raised (`PackageNotFoundError`) but it is not needed, subclass
exceptions are still accepted by pytest. Also added a message to be more
specific.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_conversion.py

@@ -140,12 +140,18 @@ def test_conversion_entry_point_string(npe1_repo, mock_npe1_pm_with_plugin):
 
 
 def test_conversion_missing():
-    with pytest.raises(ModuleNotFoundError), pytest.warns(UserWarning):
+    with pytest.raises(
+        PackageNotFoundError,
+        match="No package or entry point found with name",
+    ):
         manifest_from_npe1("does-not-exist-asdf6as987")
 
 
 def test_conversion_package_is_not_a_plugin():
-    with pytest.raises(PackageNotFoundError):
+    with pytest.raises(
+        PackageNotFoundError,
+        match="No package or entry point found with name",
+    ):
         manifest_from_npe1("pytest")