ref: add mypy stubs for django-csp (#54786)

upstream seems not super active so I sent an issue there and I'm going
to plop stubs here for now
https://github.com/mozilla/django-csp/issues/198

fixtures/stubs-for-mypy/csp/middleware.pyi

@@ -0,0 +1,8 @@
+from django.http.request import HttpRequest
+from django.http.response import HttpResponseBase
+from django.utils.deprecation import MiddlewareMixin
+
+class CSPMiddleware(MiddlewareMixin):
+    def process_request(self, request: HttpRequest) -> None: ...
+    def process_response(self, request: HttpRequest, response: HttpResponseBase) -> HttpResponseBase: ...
+    def build_policy(self, request: HttpRequest, response: HttpResponseBase) -> str: ...

pyproject.toml

@@ -95,7 +95,6 @@ module = [
     "celery.*",
     "codeowners.*",
     "confluent_kafka.*",
-    "csp.middleware.*",
     "cssselect.*",
     "django_zero_downtime_migrations.backends.postgres.schema.*",
     "docker.*",