Remove usage of deprecated Django APIs

src/pretix/helpers/http.py

@@ -38,7 +38,7 @@ class ChunkBasedFileResponse(StreamingHttpResponse):
 def get_client_ip(request):
     ip = request.META.get('REMOTE_ADDR')
     if settings.TRUST_X_FORWARDED_FOR:
-        x_forwarded_for = request.META.get('HTTP_X_FORWARDED_FOR')
+        x_forwarded_for = request.headers.get('x-forwarded-for')
         if x_forwarded_for:
             ip = x_forwarded_for.split(',')[0]
     return ip