feed: Disable feedparser's PREFERRED_XML_PARSERS

feed: Disable feedparser's PREFERRED_XML_PARSERS

Feedparser's default parser (drv_libxml2) has trouble parsing byte
streams in Python 3:

  $ python -c 'import rss2email.feed; import doctest; doctest.testmod(rss2email.feed)'
  ...
  File "rss2email/feed.py", line 319, in rss2email.feed.Feed._fetch
Failed example:
    parsed = feed._fetch()
Exception raised:
    Traceback (most recent call last):
      File "rss2email/util.py", line 61, in run
        self.result = self._target(*self._args, **self._kwargs)
      File "/.../feedparser/feedparser.py", line 3745, in parse
        saxparser.parse(source)
      File "/usr/lib64/python3.2/site-packages/drv_libxml2.py", line 270, in parse
        _d(reader.Name()),_d(reader.Value()))
      File "/usr/lib64/python3.2/site-packages/drv_libxml2.py", line 70, in _d
        return _decoder(s)[0]
      File "/usr/lib64/python3.2/encodings/utf_8.py", line 16, in decode
        return codecs.utf_8_decode(input, errors, True)
    TypeError: 'str' does not support the buffer interface

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "/usr/lib64/python3.2/doctest.py", line 1288, in __run
        compileflags, 1), test.globs)
      File "<doctest rss2email.feed.Feed._fetch[1]>", line 1, in <module>
        parsed = feed._fetch()
      File "rss2email/feed.py", line 336, in _fetch
        return f(self.url, self.etag, modified=self.modified, **kwargs)
      File "rss2email/util.py", line 76, in __call__
        time_limited_function=self) from self.error[1]
    rss2email.error.TimeoutError: error while running time limited function: 'str' does not support the buffer interface
  ...

You can reproduce the underlying exception with this minimal script:

  import io
  import xml.sax
  import xml.sax.handler

  data = b'<feed xmlns="http://www.w3.org/2005/Atom"><entry><author><name>Example author</name><email>me@example.com</email><url>http://example.com/</url></author></entry></feed>'

  source = xml.sax.xmlreader.InputSource()
  source.setByteStream(io.BytesIO(data))
  saxparser = xml.sax.make_parser(["drv_libxml2"])
  saxparser.setContentHandler(xml.sax.handler.ContentHandler())
  saxparser.parse(source)

which raises:

  Traceback (most recent call last):
    File "<stdin>", line 13, in <module>
      saxparser.parse(source)
    File "/usr/lib64/python3.2/site-packages/drv_libxml2.py", line 222, in parse
      eltName = _d(reader.Name())
    File "/usr/lib64/python3.2/site-packages/drv_libxml2.py", line 70, in _d
      return _decoder(s)[0]
    File "/usr/lib64/python3.2/encodings/utf_8.py", line 16, in decode
      return codecs.utf_8_decode(input, errors, True)
  TypeError: 'str' does not support the buffer interface

at least for libxml2-2.9.1.

By using the stdlib's default parser (instead of drv_libxml2), we can
avoid the error and get successful parsing.  If you don't have
drv_libxml2 installed, sax was already falling back on the stdlib's
default parser, so this commit will be a no-op.

Signed-off-by: W. Trevor King <wking@tremily.us>