In order to avoid messing too much with the intricacies of the existing email parsing, here is a patch for Python 3 that limits the behaviour changes to the HTTP module. It should fix the bad handling of broken header lines. As a side effect, it should also fix Issue 22233, since it bypasses the offending splitlines() call. I incorporated the test cases from my previous patches for Issue 26686 and Issue 22233.

I tried to maintain a minimal compatibility with the previous special behaviour for message/* and multipart/* message types, although I didn’t bother trying to emulate e.g. StartBoundaryNotFoundDefect (http.client.parse_headers() doesn’t pass any body to the email parser, so it won’t see any start boundaries.)