Merge multiple isinstance() calls to one (#6481)

https://docs.python.org/3/library/functions.html#isinstance

> If classinfo is a tuple of type objects (or recursively, other such
> tuples), return true if object is an instance of any of the types.

Author: jdufresne

rest_framework/fields.py

@@ -1660,7 +1660,7 @@ def to_internal_value(self, data):
         """
         if html.is_html_input(data):
             data = html.parse_html_list(data, default=[])
-        if isinstance(data, type('')) or isinstance(data, Mapping) or not hasattr(data, '__iter__'):
+        if isinstance(data, (type(''), Mapping)) or not hasattr(data, '__iter__'):
             self.fail('not_a_list', input_type=type(data).__name__)
         if not self.allow_empty and len(data) == 0:
             self.fail('empty')