replace use of imp with importlib

The imp module has been deprecated since Python 3.4 and was removed in
Python 3.12. Use importlib, the recommended replacement, instead. Code
based on the recipe for importing a source file directly in the official
docs:
https://docs.python.org/3.12/library/importlib.html#importing-a-source-file-directly.

ledgerautosync/cli.py

@@ -21,7 +21,7 @@
 
 
 import argparse
-import imp
+import importlib.util
 import logging
 import os
 import os.path
@@ -233,10 +233,12 @@ def load_plugins(config_dir):
             # Quiet loader
             import ledgerautosync.plugins  # noqa: F401
 
-            path = os.path.join(plugin_dir, plugin)
-            imp.load_source(
-                "ledgerautosync.plugins.%s" % (os.path.splitext(plugin)[0]), path
-            )
+            file_path = os.path.join(plugin_dir, plugin)
+            module_name = f"ledgerautosync.plugins.{os.path.splitext(plugin)[0]}"
+            spec = importlib.util.spec_from_file_location(module_name, file_path)
+            module = importlib.util.module_from_spec(spec)
+            sys.modules[module_name] = module
+            spec.loader.exec_module(module)
 
 
 def run(args=None, config=None):