This change fixes bug #4958 by changing zipfile in

DownloadPatchErrorBundleView to use BytesIO instead of StringIO, which

makes it work on both Python 2 and 3. Before this change, the view would fail

when Python 3 was used.
This change also adds two unit tests for DownloadPatchErrorBundleView that test

its response when there is or isn't a PatchError raised in rendering the diff.

A review request with a patch that does not apply cleanly was created.

The patch error bundle was successfully downloaded when running

the Review Board dev server on both Python 2 and 3.
Two unit tests were added for DownloadPatchErrorBundleView that test its

responses. It was verified that these unit tests fail on release-4.0.x

when Python 3 is used, and pass on both Python 2 and 3 after this change.
All unit tests in reviewboard.diffviewer.tests were run and passed on

both Python 2 and 3.