Avoid importing django.test package when not testing (#8699)

Importing anything `rest_framework` causes `django.test` to be imported.
This is because DRF registers a receiver on the
`django.test_signals.setting_changed` signal.

This is not really a problem, but it is good to avoid this because it
bloats the memory with unnecessary modules (e.g. `django.test`,
`django.core.servers.basehttp`, `socketserver`) and increases the
startup time. It also doesn't feel right to import test code into
non-test code.

Try to import the signal from a core module if possible.

Note that there's another `django.test` import in `MultiPartRenderer`,
however this import is done lazily only if the functionality is used so
can be easily avoided.

rest_framework/settings.py

@@ -19,7 +19,9 @@ REST framework settings, checking for user settings first, then falling
 back to the defaults.
 """
 from django.conf import settings
-from django.test.signals import setting_changed
+# Import from `django.core.signals` instead of the official location
+# `django.test.signals` to avoid importing the test module unnecessarily.
+from django.core.signals import setting_changed
 from django.utils.module_loading import import_string
 
 from rest_framework import ISO_8601