Locate the bundled model directory with importlib.resources instead of
pkg_resources, which current setuptools no longer provides.

--- a/perth/perth_net/__init__.py
+++ b/perth/perth_net/__init__.py
@@ -1,4 +1,4 @@
-from pkg_resources import resource_filename
-PREPACKAGED_MODELS_DIR = resource_filename(__name__, "pretrained")
+from importlib.resources import files
+PREPACKAGED_MODELS_DIR = str(files(__name__) / "pretrained")

 from .perth_net_implicit.perth_watermarker import PerthImplicitWatermarker