python: replace pkg_resources with importlib.metadata

Use of pkg_resources is discouraged in favor of importlib.resources,
importlib.metadata, and their backports.

python/pyproject.toml

@@ -21,6 +21,7 @@ classifiers = [
 dependencies = [
     "cffi>=1.0.0",
     "imap-tools",
+    "importlib_metadata;python_version<'3.8'",
     "pluggy",
     "requests",
 ]