Merge fix from 3.3 into default.

Fix issue 18889: test_sax: multiple failures on Windows desktop.

"The fix" is to tell Mercurial that the test files are binary.

Windows developers:  to get the correct line endings in your checkout,
delete Lib\test\xmltestdata, and then "hg revert" that directory.

Why the Windows buildbots didn't fail test_sax remains a mystery :-(