chore: unpin django-stubs now that we're on django 4.2

django-stubs, which provides better type hinting for Django projects,
was pinned to an old version (1.16.0) in order to support Django
versions <=4.

Now that we have upgraded edx-platform to 4.2, we can remove the
django-stubs pin, and use the latest version. This will allow us to use
type hints more aggressively, especially in the content_libraries app.

requirements/constraints.txt

@@ -60,13 +60,6 @@ django-webpack-loader==0.7.0
 # Adding pin to avoid any major upgrade
 djangorestframework<3.15.0
 
-# Date: 2023-07-19
-# The version of django-stubs we can use depends on which Django release we're using
-# 1.16.0 works with Django 3.2 through 4.1
-# Issue for unpinning: https://github.com/openedx/edx-platform/issues/35275
-django-stubs==1.16.0
-djangorestframework-stubs==3.14.0  # Pinned to match django-stubs. Remove this when we can remove the above pin.
-
 # Date: 2024-07-23
 # django-storages==1.14.4 breaks course imports
 # Two lines were added in 1.14.4 that make file_exists_in_storage function always return False,