filter-pypi-responses: stream package requests that hit the proxy

mitmproxy was buffering all requests (except pythonhosted.*), causing
pip to time out when getting big packages from custom URLs.

Setting flow.response.stream in the responseheaders hook fixes this.

pkgs/fetchPipMetadata/filter-pypi-responses.py

@@ -85,6 +85,11 @@ Response format:
 """
 
 
+def responseheaders(flow: http.HTTPFlow) -> None:
+    if "/simple/" not in flow.request.url:
+        flow.response.stream = True
+
+
 def response(flow: http.HTTPFlow) -> None:
     if not "/simple/" in flow.request.url:
         return