pip: disable cache for modified response

The pypi proxy filters packages, which can end up corrupting the pip
cache. For an example of what this would look like, see
pypi/warehouse#14457 This commit prevents the
the cache from storing our modified response by adding a 'Vary: *'
header. Judging from the caching code, this is the least intrusive
approach to prevent caching ('Cache-Control: no-cache' deletes existing
caches for example).

pkgs/fetchPipMetadata/filter-pypi-responses.py

@@ -95,3 +95,11 @@ def response(flow: http.HTTPFlow) -> None:
         print(f"removing the following files form the API response:\n  {badFiles}")
         data["files"] = list(filter(keepFile, data["files"]))
     flow.response.text = json.dumps(data)
+    # prevent the modified response from ending up in the pip cache
+    flow.response.headers["Vary"] = "*"
+    # See this comment in cachecontrol/controller.py in pip:
+    # https://tools.ietf.org/html/rfc7234#section-4.1:
+    # A Vary header field-value of "*" always fails to match.
+    # Storing such a response leads to a deserialization warning
+    # during cache lookup and is not allowed to ever be served,
+    # so storing it can be avoided.