When calling HTTPResponse.read() on a response that is:
a. not chunked
b. contains no content-length header
the underlying socket (referenced by self.fp) will never get closed (through self.close())

The offending code is at the bottom of the read() function:
        s = self.fp.read(amt)
        if self.length is not None:
            self.length -= len(s)
            if not self.length:
                self.close()
        return s
As seen, if self.length is None, even when the server closes the connection (causing self.fp.read to return ''), the socket will not get closed.

btw, this may be the cause of Issue15633 (http://bugs.python.org/issue15633)