[Resolves #582] update imp to importlib (#1092)

The imp.py library has been deprecated since ver 3.4 in favor of
importlib[1]. This updates to using importlib.

[1] https://docs.python.org/3/library/imp.html

integration-tests/steps/templates.py

@@ -1,9 +1,9 @@
 from behave import *
 import os
-import imp
 import yaml
 
 from botocore.exceptions import ClientError
+from importlib.machinery import SourceFileLoader
 from sceptre.plan.plan import SceptrePlan
 from sceptre.context import SceptreContext
 from sceptre.cli.helpers import CfnYamlLoader
@@ -112,7 +112,7 @@ def step_impl(context, filename):
         context.sceptre_dir, "templates", filename
     )
 
-    module = imp.load_source("template", filepath)
+    module = SourceFileLoader("template", filepath).load_module()
     body = module.sceptre_handler({})
     for template in context.output.values():
         assert body == template

sceptre/template.py

@@ -7,14 +7,14 @@
 and implements methods for uploading it to S3.
 """
 
-import imp
 import logging
 import os
 import sys
 import threading
 import traceback
 
 import botocore
+from importlib.machinery import SourceFileLoader
 from jinja2 import Environment
 from jinja2 import FileSystemLoader
 from jinja2 import StrictUndefined
@@ -174,7 +174,7 @@ def _call_sceptre_handler(self):
         if not os.path.isfile(self.path):
             raise IOError("No such file or directory: '%s'", self.path)
 
-        module = imp.load_source(self.name, self.path)
+        module = SourceFileLoader(self.name, self.path).load_module()
 
         try:
             body = module.sceptre_handler(self.sceptre_user_data)