Fix artifact download to deal with changes on the Github side. (#4769)

It looks like the Github backend used to include the HTTP status as an
actual header, in addition to sending it as a status.  But now they
don't seem to be doing it.

The difference between requestBlobAndCheck and requestBlob, looking at
<https://github.com/PyGithub/PyGithub/blob/master/github/Requester.py>,
is that the former calls requestBlob, then does JSON parsing on the
body, checks that the status is not an error, then returns just the
headers and response body to the caller.

Since we do actually care about the status, and don't care about the
response body and the JSON-decoding that requestBlobAndCheck does, we
can just use requestBlob directly.  That lets us examine the status in
the way we want.
diff --git a/scripts/helpers/github_fetch_artifacts.py b/scripts/helpers/github_fetch_artifacts.py
index 95031fe..b146fae 100644
--- a/scripts/helpers/github_fetch_artifacts.py
+++ b/scripts/helpers/github_fetch_artifacts.py
@@ -53,10 +53,10 @@
     url = self.archive_download_url
     logging.info('Fetching: %r' % url)
 
-    headers, _ = self._requester.requestBlobAndCheck('GET', url)
+    status, headers, _ = self._requester.requestBlob('GET', url)
 
-    if 'status' not in headers or headers['status'] != '302 Found':
-        raise Exception('Expected a redirect during blob download but got %r.' % headers)
+    if status != 302:
+        raise Exception('Expected a redirect during blob download but got status %d, headers %r.' % (status, headers))
 
     response = requests.get(headers['location'])
     response.raise_for_status()