Update jsonnet callbacks to provide byte strings

This is necessary for Jsonnet >= 0.19.0

commodore/postprocess/jsonnet.py

@@ -32,7 +32,7 @@ def _try_path(basedir: P, rel: str):
     if not full_path.is_file():
         return full_path.name, None
     with open(full_path, encoding="utf-8") as f:
-        return full_path.name, f.read()
+        return full_path.name, f.read().encode("utf-8")
 
 
 def _import_callback_with_searchpath(search: Iterable[P], basedir: P, rel: str):

tests/test_postprocess.py

@@ -399,7 +399,7 @@ def test_postprocess_jsonnet_try_path(tmp_path, full_rel):
     path, contents = jsonnet_pp._try_path(tmp_path, rel)
 
     assert path == testf.name
-    assert contents == "Test"
+    assert contents == b"Test"
 
 
 @pytest.mark.parametrize(
@@ -430,7 +430,7 @@ def test_postprocess_jsonnet_import_cb(tmp_path, basedir, floc):
     path, contents = jsonnet_pp._import_cb(tmp_path, bdir, "test.txt")
 
     assert path == "test.txt"
-    assert contents == f"Test {tmp_path / floc}"
+    assert contents == f"Test {tmp_path / floc}".encode("utf-8")
 
 
 def test_postprocess_jsonnet_import_cb_notfound(tmp_path):