Ignore ast package in codecov

ast package did not have any tests previously and a new test
was added. The codecov tool computes that code coverage has dropped by
15% even though a new test was added.

Adding ast to ignore list as a workaround so that the pull request can
be merged.

Thanks goccy for the suggestion.

.codecov.yml

@@ -26,3 +26,6 @@ comment:
   layout: "header,diff"
   behavior: default
   require_changes: no
+
+ignore:
+  - ast