fix: Switch from imp to importlib.

The import in `grader.py` was just not being used so I dropped it.  In
jailedgrader.py I replaced the implementation with the sourcefile loader
from importlib which is still around.  The `imp` module is deprecated
and was removed in Python 3.12

With these changes the code is compatible with Python 3.8 and 3.12.

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         os:
         - ubuntu-20.04
-        python-version: ['3.8', '3.11']
+        python-version: ['3.8', '3.11', '3.12']
     steps:
     - uses: actions/checkout@v2
     - name: setup python

xqueue_watcher/grader.py

@@ -2,7 +2,6 @@
 Implementation of a grader compatible with XServer
 """
 import html
-import imp
 import sys
 import time
 import json

xqueue_watcher/jailedgrader.py

@@ -4,7 +4,7 @@
 import codecs
 import os
 import sys
-import imp
+import importlib
 import json
 import random
 import gettext
@@ -122,7 +122,8 @@ def grade(self, grader_path, grader_config, submission):
         # Import the grader, straight from the original file.  (It probably isn't in
         # sys.path, and we may be in a long running gunicorn process, so we don't
         # want to add stuff to sys.path either.)
-        grader_module = imp.load_source("grader_module", str(grader_path))
+        sf_loader = importlib.machinery.SourceFileLoader("grader_module", str(grader_path))
+        grader_module = sf_loader.load_module()
         grader = grader_module.grader
 
         # Preprocess for grader-specified errors