Use backport importlib_resources for older pythons

Lib/gfsubsets/__init__.py

@@ -7,12 +7,14 @@
 import re
 import sys
 import warnings
-from importlib import resources
+# importlib.resources.files only available since Python>3.9,
+# temporarily use backport
+from importlib_resources import files
 from fontTools import ttLib
 
 from . import subsets
 
-nam_files = resources.files("gfsubsets.data")
+nam_files = files("gfsubsets.data")
 
 
 class NamFileDict(dict):

Lib/gfsubsets/subsets.py

@@ -15,10 +15,12 @@
 # limitations under the License.
 #
 import os
-from importlib import resources
+# importlib.resources.files only available since Python>3.9,
+# temporarily use backport
+from importlib_resources import files
 
 subset_files = [resource.name for resource 
-                in resources.files("gfsubsets.data").iterdir()
+                in files("gfsubsets.data").iterdir()
                 if resource.name.endswith("_unique-glyphs.nam")]
 
 SUBSETS = sorted(

pyproject.toml

@@ -19,6 +19,15 @@ authors = [
     {name = 'Garret Rieger', email = "[email protected]" },
     {name = 'Roderick Sheeter', email = "[email protected]" }
 ]
+dependencies = [
+    "importlib_resources"  # Backport for Python < 3.9
+]
+
+[project.optional-dependencies]
+dev = [
+    "youseedee"
+]
+
 
 [tool.setuptools.package-dir]
 gfsubsets = "Lib/gfsubsets"