Use callable() builtin function over isinstance(..., Callable)

This is slightly more compact, but the main motivation is to work
around the following mypy bug:

python/mypy#3060

model_utils/fields.py

@@ -1,7 +1,6 @@
 import secrets
 import uuid
 import warnings
-from collections.abc import Callable
 
 from django.conf import settings
 from django.core.exceptions import ValidationError
@@ -346,7 +345,7 @@ def __init__(self, editable=False, max_length=128, factory=None, **kwargs):
             non-callable value for factory is not supported.
         """
 
-        if factory is not None and not isinstance(factory, Callable):
+        if factory is not None and not callable(factory):
             raise TypeError("'factory' should either be a callable or 'None'")
         self._factory = factory