Check vars against Mapping in DEBUG (#66355)

When ANSIBLE_DEBUG=1, vars are VarsWithSources now which is a Mapping.
Check vars against Mapping instead of dict.

lib/ansible/template/__init__.py

@@ -506,8 +506,8 @@ class Templar:
         are being changed.
         '''
 
-        if not isinstance(variables, dict):
-            raise AnsibleAssertionError("the type of 'variables' should be a dict but was a %s" % (type(variables)))
+        if not isinstance(variables, Mapping):
+            raise AnsibleAssertionError("the type of 'variables' should be a Mapping but was a %s" % (type(variables)))
         self._available_variables = variables
         self._cached_result = {}