bpo-1812: Fix newline conversion when doctest.testfile loads from a package whose loader has a get_data method (GH-17385)

This pull request fixes the newline conversion bug originally reported in bpo-1812. When that issue was originally submitted, the open builtin did not default to universal newline mode; now it does, which makes the issue fix simpler, since the only code path that needs to be changed is the one in doctest._load_testfile where the file is loaded from a package whose loader has a get_data method.