712 Handle EOFErrror with malformed file

[nicholasserra]

Related to pypi/warehouse#8815 and pypi/warehouse#8378

Fixes #712
Handle EOFError when uploading malformed tar.gz file. Uses example tar.gz file from issue.

This is a stab at fixing this issue, although I don't love a couple things here:

• Catching this error within twine, as this is mainly an issue in the sdist package. Is the catch appropriate here?
• Adding malformed.tar.gz into version control for the test. Doesn't feel great, but maybe it's fine?

Maybe this is all okay. Let me know if there's better opinions or direction here. Thanks! cc @di