Replace imp with importlib

This project uses the imp module which has been deprecated since Python 3.4 and set for removal in 3.12:
• Raised PendingDeprecationWarning since 3.4 (2014)
• Raised DeprecationWarning since 3.5 (2015)
• Updated DeprecationWarning to say removal in 3.12 since 3.10 (2021)
• Removal planned for 3.12 (2023)

This change removes the dependency on imp in favour of importlib.

Co-authored-by: @jbkkd
Inspired by: @mgorny

https://github.com/jazzband/django-configurations/issues/358

configurations/importer.py

@@ -1,4 +1,5 @@
-import imp
+import importlib.util
+from importlib.machinery import PathFinder
 import logging
 import os
 import sys
@@ -126,25 +127,30 @@ class ConfigurationImporter:
                                                       self.name))
                 self.logger.debug(stylize(message))
 
-    def find_module(self, fullname, path=None):
+    def find_spec(self, fullname, path=None, target=None):
         if fullname is not None and fullname == self.module:
-            module = fullname.rsplit('.', 1)[-1]
-            return ConfigurationLoader(self.name,
-                                       imp.find_module(module, path))
+            spec = PathFinder.find_spec(fullname, path)
+            if spec is not None:
+                return importlib.machinery.ModuleSpec(spec.name,
+                    ConfigurationLoader(self.name, spec),
+                    origin=spec.origin)
         return None
 
 
 class ConfigurationLoader:
 
-    def __init__(self, name, location):
+    def __init__(self, name, spec):
         self.name = name
-        self.location = location
+        self.spec = spec
 
     def load_module(self, fullname):
         if fullname in sys.modules:
             mod = sys.modules[fullname]  # pragma: no cover
         else:
-            mod = imp.load_module(fullname, *self.location)
+            mod = importlib.util.module_from_spec(self.spec)
+            sys.modules[fullname] = mod
+            self.spec.loader.exec_module(mod)
+
         cls_path = '{0}.{1}'.format(mod.__name__, self.name)
 
         try: