Removed usage of deprecated is_iterable util (carltongibson#1642)

Django announced [1] that the `django.utils.itercompat` module is
deprecated since Django 5.1.

Instead, the same behavior can be achieved with an instance comparison
against `Iterable` [2].

[1] https://docs.djangoproject.com/en/dev/releases/5.1/#features-deprecated-in-5-1
[2] https://docs.python.org/3/library/collections.abc.html#collections.abc.Iterable

django_filters/filters.py

@@ -1,4 +1,5 @@
 from collections import OrderedDict
+from collections.abc import Iterable
 from datetime import timedelta
 from itertools import chain
 
@@ -7,7 +8,6 @@
 from django.db.models import Q
 from django.db.models.constants import LOOKUP_SEP
 from django.forms.utils import pretty_name
-from django.utils.itercompat import is_iterable
 from django.utils.timezone import now
 from django.utils.translation import gettext_lazy as _
 
@@ -776,14 +776,14 @@ def normalize_fields(cls, fields):
             return OrderedDict(fields)
 
         # convert iterable of values => iterable of pairs (field name, param name)
-        assert is_iterable(
-            fields
+        assert isinstance(
+            fields, Iterable
         ), "'fields' must be an iterable (e.g., a list, tuple, or mapping)."
 
         # fields is an iterable of field names
         assert all(
             isinstance(field, str)
-            or is_iterable(field)
+            or isinstance(field, Iterable)
             and len(field) == 2  # may need to be wrapped in parens
             for field in fields
         ), "'fields' must contain strings or (field name, param name) pairs."