bpo-1812: Fix newline conversion when doctest.testfile loads from a package whose...

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.
(cherry picked from commit e0b8101492f6c61dee831425b4d3dae39a953599)

Co-authored-by: Peter Donis <peterdonis@alum.mit.edu>