dispatch: cope with sys.version being unicode on Python 3

dispatch: allow testedwith to be bytes or str

ui: fix ui.traceback on Python 3

ui: fix opts labeling on ui.warn et al for Python 3 This is a step towards fixing extension load warnings on Python 3. Note that I suspect there are still some bugs in this area and that things like color won't work, but the code at least executes and prints text to the console correctly now.

config: add sanity assert that files are opened as binary This helps with some debugging in Python 3, and shouldn't hurt anything in Python 2. The unusual construction using getattr is done so that StringIO/BytesIO instances can be used as well as real files.

update: for "noconflict" updates, print "conflicting changes" on conflict With experimental.updatecheck=noconflict, if the update is aborted because of conlicts, "uncommitted changes" is not quite accurate. Let's use "conflicting changes" instead. Also fix the hint to recomment --clean, not --merge, since that's what we do for other failed updates.

tests: fix test-update-branches to remove non-conflicting file I was clearly very sloppy when I wrote the test case for experimental.updatecheck=noconflict. The test case that checks that one can move between commits with a removed file was deleting a file that was modified between the source and target commits, which resulted in a "change/delete" conflict. Since that is a conlict, the update correctly failed. Let's fix the test by removing a file that is not modified between the two commits.