Tempfile.py: stop buildbot warning about using deprecated xreadlines.
Tempfile.py: stop buildbot warning about using deprecated xreadlines.

The slightly odd behavior (the validity of passing a sizehint depends on the
type of self._file) was kept to avoid breaking code that depends on it.
Test_tempfile.test_xreadlines passes (along with everything else).

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator