Update sceptre to use importlib (#1240)

importlib was introducted in python 3.8 but we want to continue
support for python 3.7 therefore we need to use pkg_resources for
python 3.7 while using importlib for python >= 3.8

sceptre/config/reader.py

@@ -12,13 +12,12 @@
 import datetime
 import fnmatch
 import logging
+import sys
+import yaml
+
 from os import environ, path, walk
 from typing import Set
-
-from pkg_resources import iter_entry_points
 from pathlib import Path
-import yaml
-
 from jinja2 import Environment
 from jinja2 import StrictUndefined
 from jinja2 import FileSystemLoader
@@ -137,6 +136,19 @@ def __init__(self, context):
 
         self.templating_vars = {"var": self.context.user_variables}
 
+    @staticmethod
+    def _iterate_entry_points(group):
+        """
+        Helper to determine whether to use pkg_resources or importlib.metadata.
+        https://docs.python.org/3/library/importlib.metadata.html
+        """
+        if sys.version_info < (3, 10):
+            from pkg_resources import iter_entry_points
+            return iter_entry_points(group)
+        else:
+            from importlib.metadata import entry_points
+            return entry_points(group=group)
+
     def _add_yaml_constructors(self, entry_point_groups):
         """
         Adds PyYAML constructor functions for all classes found registered at
@@ -171,7 +183,8 @@ def class_constructor(loader, node):
             return class_constructor
 
         for group in entry_point_groups:
-            for entry_point in iter_entry_points(group):
+
+            for entry_point in self._iterate_entry_points(group):
                 # Retrieve name and class from entry point
                 node_tag = u'!' + entry_point.name
                 node_class = entry_point.load()

sceptre/template.py

@@ -10,9 +10,9 @@
 import logging
 import threading
 import botocore
+import sys
 
 from sceptre.exceptions import TemplateHandlerNotFoundError
-from pkg_resources import iter_entry_points
 
 
 class Template(object):
@@ -234,6 +234,19 @@ def _bucket_region(self, bucket_name):
     def _domain_from_region(region):
         return "com.cn" if region.startswith("cn-") else "com"
 
+    @staticmethod
+    def _iterate_entry_points(group, name):
+        """
+        Helper to determine whether to use pkg_resources or importlib.metadata.
+        https://docs.python.org/3/library/importlib.metadata.html
+        """
+        if sys.version_info < (3, 10):
+            from pkg_resources import iter_entry_points
+            return iter_entry_points(group, name)
+        else:
+            from importlib.metadata import entry_points
+            return entry_points(group=group, name=name)
+
     def _get_handler_of_type(self, type):
         """
         Gets a TemplateHandler type from the registry that can be used to get a string
@@ -246,7 +259,7 @@ def _get_handler_of_type(self, type):
         if not self._registry:
             self._registry = {}
 
-            for entry_point in iter_entry_points("sceptre.template_handlers", type):
+            for entry_point in self._iterate_entry_points("sceptre.template_handlers", type):
                 self._registry[entry_point.name] = entry_point.load()
 
         if type not in self._registry: