Do not use mutable data structures for argument defaults

[reinhash]

When stepping through the code in filters.py, I stumbled upon two instance where a function uses a dictionary as a default value.
While I think this is currently not a big problem and a low prio issue, any mutation to the variable could lead to undefined behaviour. Ruff has a great document concerning this.

How to reproduce:

uv run ruff check .venv/lib/python3.12/site-packages/rest_framework/* | grep B006

Here the lines where I found the problem:

• django-rest-framework/rest_framework/filters.py

  Line 252 in 2ae8c11

  def get_default_valid_fields(self, queryset, view, context={}):

• django-rest-framework/rest_framework/filters.py

  Line 289 in 2ae8c11

  def get_valid_fields(self, queryset, view, context={}):